diff --git a/Cargo.lock b/Cargo.lock
index 9c720b9..ade23a0 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1543,6 +1543,7 @@ dependencies = [
  "log",
  "serde",
  "serde_json",
+ "sys-locale",
  "toml",
 ]
 
@@ -3241,6 +3242,15 @@ dependencies = [
  "unicode-ident",
 ]
 
+[[package]]
+name = "sys-locale"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8eab9a99a024a169fe8a903cf9d4a3b3601109bcc13bd9e3c6fff259138626c4"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "tempfile"
 version = "3.27.0"
diff --git a/Cargo.toml b/Cargo.toml
index a1ce6a5..2e0e895 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -43,6 +43,8 @@ eframe = { version = "0.36", default-features = false }
 egui_kittest = "0.36"
 rfd = "0.17"
 arboard = { version = "3.6", default-features = false }
+# The desktop's preferred language, for the first-run interface language (native only).
+sys-locale = "0.3"
 
 [workspace.lints.rust]
 unsafe_code = "forbid"
diff --git a/apps/gridcraft/src/main.rs b/apps/gridcraft/src/main.rs
index 1c7233a..6f91741 100644
--- a/apps/gridcraft/src/main.rs
+++ b/apps/gridcraft/src/main.rs
@@ -79,11 +79,16 @@ fn load_prefs(app: &mut SheetApp) {
     if !prefs_enabled() {
         return;
     }
+    // First run (or a ui.json without a usable language): follow the desktop's language.
+    app.ui.language = gridcraft_ui_egui::i18n::Language::system();
     let Some(dir) = config_dir() else { return };
     if let Ok(b) = std::fs::read(dir.join("ui.json"))
-        && let Ok(ui) = serde_json::from_slice(&b)
+        && let Ok(ui) = serde_json::from_slice::<gridcraft_ui_egui::UiState>(&b)
     {
+        let saved = serde_json::from_slice(&b).ok().and_then(|v| gridcraft_ui_egui::i18n::saved_language(&v));
+        let language = saved.unwrap_or(app.ui.language);
         app.ui = ui;
+        app.ui.language = language;
     }
     if let Ok(b) = std::fs::read(dir.join("prefs.json"))
         && let Ok(p) = serde_json::from_slice(&b)
diff --git a/crates/core/src/date.rs b/crates/core/src/date.rs
index b824434..81fa6c5 100644
--- a/crates/core/src/date.rs
+++ b/crates/core/src/date.rs
@@ -105,7 +105,7 @@ pub fn serial_from_ymd(sys: DateSystem, year: i64, month: i64, day: i64) -> Opti
 
 /// Date/time from a serial. `None` for negative or too-large serials.
 pub fn datetime_from_serial(sys: DateSystem, serial: f64) -> Option<DateTime> {
-    if !serial.is_finite() || serial < 0.0 || serial >= 2_958_466.0 {
+    if !serial.is_finite() || !(0.0..2_958_466.0).contains(&serial) {
         return None;
     }
     let mut day = serial.floor() as i64;
diff --git a/crates/functions/src/math.rs b/crates/functions/src/math.rs
index 8196f5a..4d9eb45 100644
--- a/crates/functions/src/math.rs
+++ b/crates/functions/src/math.rs
@@ -487,7 +487,7 @@ fn integer_args(a: &[Arg]) -> R<Vec<u64>> {
     v.iter()
         .map(|x| {
             let t = x.trunc();
-            if t < 0.0 || t >= 9.007_199_254_740_992e15 { Err(CellError::Num) } else { Ok(t as u64) }
+            if !(0.0..9.007_199_254_740_992e15).contains(&t) { Err(CellError::Num) } else { Ok(t as u64) }
         })
         .collect()
 }
diff --git a/crates/functions/src/stat.rs b/crates/functions/src/stat.rs
index abdb4e7..51a5f32 100644
--- a/crates/functions/src/stat.rs
+++ b/crates/functions/src/stat.rs
@@ -1185,7 +1185,7 @@ fn binom_dist_range(a: &[Arg], _: C) -> R<Value> {
     let p = num(a, 1)?;
     let s1 = num(a, 2)?.trunc();
     let s2 = if has(a, 3) { num(a, 3)?.trunc() } else { s1 };
-    if n < 0.0 || n > 1e15 || !(0.0..=1.0).contains(&p) || s1 < 0.0 || s1 > n || s2 < s1 || s2 > n {
+    if !(0.0..=1e15).contains(&n) || !(0.0..=1.0).contains(&p) || s1 < 0.0 || s1 > n || s2 < s1 || s2 > n {
         return Err(CellError::Num);
     }
     if s2 - s1 > 10_000.0 {
diff --git a/crates/functions/src/util.rs b/crates/functions/src/util.rs
index 5296c48..14c6279 100644
--- a/crates/functions/src/util.rs
+++ b/crates/functions/src/util.rs
@@ -51,7 +51,7 @@ pub(crate) fn arg(args: &[Arg], i: usize) -> R<&Arg> {
 
 /// Whether argument `i` was supplied (present and not an empty literal like `F(1,)`).
 pub(crate) fn has(args: &[Arg], i: usize) -> bool {
-    matches!(args.get(i), Some(a) if !(matches!(a.value, Value::Empty) && !a.from_ref))
+    matches!(args.get(i), Some(a) if !matches!(a.value, Value::Empty) || a.from_ref)
 }
 
 /// Number coercion for a scalar parameter: empty = 0, TRUE = 1, numeric text parses.
diff --git a/crates/numfmt/src/render.rs b/crates/numfmt/src/render.rs
index 363dec1..79fe490 100644
--- a/crates/numfmt/src/render.rs
+++ b/crates/numfmt/src/render.rs
@@ -58,7 +58,7 @@ pub(crate) fn render_general(sec: &Section, v: f64, width: usize) -> Out {
 
 /// Renders a date/time. `None` when the serial is outside the supported range.
 pub(crate) fn render_date(sec: &Section, v: f64, sys: DateSystem) -> Option<Out> {
-    if !v.is_finite() || v < 0.0 || v >= 2_958_466.0 {
+    if !v.is_finite() || !(0.0..2_958_466.0).contains(&v) {
         return None;
     }
     let sub_digits = sec.toks.iter().filter_map(|t| if let Tok::Date(DatePart::SubSec(n)) = t { Some(*n) } else { None }).max().unwrap_or(0).min(3);
diff --git a/crates/ui-egui/Cargo.toml b/crates/ui-egui/Cargo.toml
index 01d7c00..ee77578 100644
--- a/crates/ui-egui/Cargo.toml
+++ b/crates/ui-egui/Cargo.toml
@@ -20,6 +20,11 @@ image = { workspace = true }
 [dev-dependencies]
 egui_kittest = { workspace = true, features = ["wgpu", "snapshot"] }
 
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+# The desktop's preferred language, for the first-run interface language. The web build has no
+# system locale, so the crate is native-only (see i18n::Language::system).
+sys-locale = { workspace = true }
+
 [build-dependencies]
 serde_json = { workspace = true }
 toml = "1"
diff --git a/crates/ui-egui/examples/snapshot.rs b/crates/ui-egui/examples/snapshot.rs
index 28fba9e..f384d18 100644
--- a/crates/ui-egui/examples/snapshot.rs
+++ b/crates/ui-egui/examples/snapshot.rs
@@ -1,7 +1,7 @@
 //! Renders the whole GridCraft window offscreen (wgpu, no window) to a PNG.
 //!
 //! `cargo run --release -p gridcraft-ui-egui --example snapshot -- [--sample sales] [--in file.xlsx]
-//!  [--size 1440x900] [--scale 2] [--cmd 'id={json}']... [--tab Insert] [--dark]
+//!  [--size 1440x900] [--scale 2] [--cmd 'id={json}']... [--tab Insert] [--dark] [--language ja]
 //!  [--dialog 'name={json}'] out.png`
 
 use gridcraft_engine::Session;
@@ -20,6 +20,7 @@ fn main() {
     let mut pane: Option<String> = None;
     let mut chart: Option<u32> = None;
     let mut dialog: Option<String> = None;
+    let mut language: Option<String> = None;
     let mut out = "snapshot.png".to_string();
     let mut i = 0;
     while i < args.len() {
@@ -65,6 +66,10 @@ fn main() {
                 i += 1;
                 dialog = args.get(i).cloned();
             }
+            "--language" => {
+                i += 1;
+                language = args.get(i).cloned();
+            }
             other => out = other.to_string(),
         }
         i += 1;
@@ -93,6 +98,11 @@ fn main() {
             {
                 let mut app = SheetApp::new(session, Default::default());
                 app.ui.dark = dark;
+                if let Some(code) = &language
+                    && let Some(l) = gridcraft_ui_egui::i18n::Language::parse(code)
+                {
+                    app.ui.language = l;
+                }
                 if let Some(t) = tab {
                     app.ui.ribbon_tab = t;
                 }
diff --git a/crates/ui-egui/src/control.rs b/crates/ui-egui/src/control.rs
index 902ec25..b1733fd 100644
--- a/crates/ui-egui/src/control.rs
+++ b/crates/ui-egui/src/control.rs
@@ -155,16 +155,19 @@ pub fn handle(app: &mut SheetApp, ctx: &egui::Context, method: &str, p: &Json) -
         }
         "engine.commands" => {
             let mut v: Vec<Json> = app.session.commands().into_iter().map(|c| serde_json::to_value(c).unwrap_or_default()).collect();
-            for (id, label) in [
-                ("ui.ribbonTab", "Ribbon Tab"),
-                ("view.formulaBar", "Formula Bar"),
-                ("view.collapseRibbon", "Collapse Ribbon"),
-                ("view.darkMode", "Dark Mode"),
-                ("view.theme", "Display Theme"),
-                ("view.zoom100", "100%"),
-                ("ui.dialog", "Open Dialog"),
+            for (id, label, params) in [
+                ("ui.ribbonTab", "Ribbon Tab", ""),
+                ("view.formulaBar", "Formula Bar", ""),
+                ("view.collapseRibbon", "Collapse Ribbon", ""),
+                ("view.darkMode", "Dark Mode", ""),
+                ("view.theme", "Display Theme", ""),
+                ("view.zoom100", "100%", ""),
+                ("ui.dialog", "Open Dialog", ""),
+                ("app.language.set", "Interface Language", "{language: \"en\"|\"ja\"} (alias: code)"),
+                ("app.language.english", "Interface Language: English", ""),
+                ("app.language.japanese", "Interface Language: Japanese", ""),
             ] {
-                v.push(json!({"id": id, "label": label, "enabled": true, "ui": true}));
+                v.push(json!({"id": id, "label": label, "params": params, "enabled": true, "ui": true}));
             }
             ok(Json::Array(v))
         }
diff --git a/crates/ui-egui/src/formula_bar.rs b/crates/ui-egui/src/formula_bar.rs
index dace534..826c996 100644
--- a/crates/ui-egui/src/formula_bar.rs
+++ b/crates/ui-egui/src/formula_bar.rs
@@ -169,7 +169,7 @@ pub fn editor_widget(app: &mut SheetApp, ui: &mut egui::Ui, id: egui::Id, font:
             ed.caret_to_end = false;
             ed.sync_caret = true;
         }
-        if !ed.autocomplete.is_empty() && (tab || (enter && false)) {
+        if !ed.autocomplete.is_empty() && tab {
             ed.accept_autocomplete();
         } else if enter {
             action = Some((if shift { -1 } else { 1 }, 0, false, false));
diff --git a/crates/ui-egui/src/i18n.rs b/crates/ui-egui/src/i18n.rs
new file mode 100644
index 0000000..18fda08
--- /dev/null
+++ b/crates/ui-egui/src/i18n.rs
@@ -0,0 +1,449 @@
+//! Interface translations. Command ids and document text stay stable; only interface chrome is
+//! translated. Untranslated labels fall back to English, so coverage can grow incrementally.
+//!
+//! The interface language is a preference ([`crate::UiState::language`]): on first run it defaults
+//! to the system's language ([`Language::system`]), and the View tab switches it by hand.
+//!
+//! No Japanese font is bundled and none is installed here: the CJK faces [`crate::theme`] appends
+//! to every family already cover kana and kanji, so Japanese interface text renders without help.
+
+use serde::{Deserialize, Serialize};
+
+#[derive(Clone, Copy, Debug, Default, PartialEq, Eq, Serialize, Deserialize)]
+#[serde(rename_all = "lowercase")]
+pub enum Language {
+    #[default]
+    En,
+    Ja,
+}
+
+impl Language {
+    pub const ALL: [Self; 2] = [Self::En, Self::Ja];
+
+    /// The language's own name, shown in the switcher.
+    pub fn name(self) -> &'static str {
+        match self {
+            Self::En => "English",
+            Self::Ja => "日本語",
+        }
+    }
+
+    /// The language's code, as saved in `ui.json` and taken by `app.language.set`.
+    pub fn code(self) -> &'static str {
+        match self {
+            Self::En => "en",
+            Self::Ja => "ja",
+        }
+    }
+
+    pub fn parse(code: &str) -> Option<Self> {
+        match code {
+            "en" => Some(Self::En),
+            "ja" => Some(Self::Ja),
+            _ => None,
+        }
+    }
+
+    /// A language tag (`ja`, `ja-JP`, `en_US`) reduced to a supported language. Native only: the web
+    /// build has no system locale to read and tests exercise it on the host.
+    #[cfg(not(target_arch = "wasm32"))]
+    fn from_tag(tag: &str) -> Option<Self> {
+        Self::parse(&tag.split(['-', '_']).next().unwrap_or("").to_ascii_lowercase())
+    }
+
+    /// The system's preferred interface language, best effort; English when it is unknown or
+    /// unsupported. Used as the first-run default while nothing is saved.
+    pub fn system() -> Self {
+        #[cfg(not(target_arch = "wasm32"))]
+        {
+            sys_locale::get_locale().and_then(|tag| Self::from_tag(&tag)).unwrap_or(Self::En)
+        }
+        #[cfg(target_arch = "wasm32")]
+        {
+            Self::En
+        }
+    }
+
+    /// Translates an interface label; untranslated labels fall back to English.
+    ///
+    /// A label that needs two translations (Home's text "Orientation" vs Page Layout's print
+    /// "Orientation") is keyed `context|label`: English shows only the part after the `|`.
+    pub fn tr(self, text: &str) -> &str {
+        if self == Self::Ja
+            && let Some((_, japanese)) = JAPANESE.iter().find(|(english, _)| *english == text)
+        {
+            return japanese;
+        }
+        text.rsplit_once('|').map_or(text, |(_, label)| label)
+    }
+}
+
+/// Deserializes a saved language leniently: an unknown or malformed value (a language removed in a
+/// later version, a hand-edited `ui.json`) reads as English instead of failing the whole `UiState`,
+/// which would reset every other preference with it.
+pub fn lenient<'de, D: serde::Deserializer<'de>>(d: D) -> Result<Language, D::Error> {
+    #[derive(Deserialize)]
+    #[serde(untagged)]
+    enum Raw {
+        Code(String),
+        Other(serde::de::IgnoredAny),
+    }
+    Ok(match Raw::deserialize(d)? {
+        Raw::Code(code) => Language::parse(&code).unwrap_or_default(),
+        Raw::Other(_) => Language::default(),
+    })
+}
+
+/// The language a saved `ui.json` asks for, if it names a supported one. The desktop app uses it
+/// to tell "saved English" from "nothing usable saved" (first run, an older file, an unknown
+/// value), which starts from [`Language::system`] instead.
+pub fn saved_language(ui_json: &serde_json::Value) -> Option<Language> {
+    ui_json.get("language").and_then(serde_json::Value::as_str).and_then(Language::parse)
+}
+
+/// egui context-data key holding the language of the frame being drawn. Widgets that only get a
+/// `Ui` (not the app) read it to translate their labels; [`set_current`] writes it once per frame.
+fn lang_key() -> egui::Id {
+    egui::Id::new("gridcraft.language")
+}
+
+/// The language the current frame is drawing in. `En` when unset.
+pub fn current(ctx: &egui::Context) -> Language {
+    ctx.data(|d| d.get_temp(lang_key())).unwrap_or_default()
+}
+
+/// Sets the language for the current frame; call once per frame before anything is drawn.
+pub fn set_current(ctx: &egui::Context, language: Language) {
+    ctx.data_mut(|d| d.insert_temp(lang_key(), language));
+}
+
+/// English label → Japanese. Keys are the exact interface strings (command ids, file names and
+/// document text are never translated); anything absent falls back to English.
+const JAPANESE: &[(&str, &str)] = &[
+    ("Paste", "貼り付け"),
+    ("Paste Values", "値の貼り付け"),
+    ("Paste Formulas", "数式の貼り付け"),
+    ("Paste Formatting", "書式の貼り付け"),
+    ("Transpose", "行列を入れ替え"),
+    ("Paste Link", "リンクの貼り付け"),
+    ("Paste Special…", "形式を選択して貼り付け…"),
+    ("Cut (⌘X)", "切り取り (⌘X)"),
+    ("Copy (⌘C)", "コピー (⌘C)"),
+    ("Format Painter (double-click to keep it on)", "書式のコピー/貼り付け (ダブルクリックで固定)"),
+    ("Bold (⌘B)", "太字 (⌘B)"),
+    ("Italic (⌘I)", "斜体 (⌘I)"),
+    ("Underline (⌘U)", "下線 (⌘U)"),
+    ("Double Underline", "二重下線"),
+    ("Strikethrough", "取り消し線"),
+    ("Underline", "下線"),
+    ("Increase Font Size", "フォントの拡大"),
+    ("Decrease Font Size", "フォントの縮小"),
+    ("Wrap Text", "折り返して全体を表示"),
+    ("Merge & Center", "セルを結合して中央揃え"),
+    ("Merge Cells", "セルの結合"),
+    ("Merge Across", "横方向に結合"),
+    ("Unmerge Cells", "セル結合の解除"),
+    ("Fill Color", "塗りつぶしの色"),
+    ("Font Color", "フォントの色"),
+    ("Borders", "罫線"),
+    ("Top\nAlign", "上揃え"),
+    ("Middle\nAlign", "中央揃え (縦)"),
+    ("Bottom\nAlign", "下揃え"),
+    ("Left", "左"),
+    ("Right", "右"),
+    ("Center", "中央"),
+    ("Horizontal", "水平"),
+    ("Vertical Text", "縦書き"),
+    ("Decrease Indent", "インデントを減らす"),
+    ("Increase Indent", "インデントを増やす"),
+    ("Orientation", "方向"),
+    ("Page Layout|Orientation", "印刷の向き"),
+    ("Angle Counterclockwise", "左回りに回転"),
+    ("Angle Clockwise", "右回りに回転"),
+    ("Rotate Text Up", "文字列を上に回転"),
+    ("Rotate Text Down", "文字列を下に回転"),
+    ("Accounting Number Format", "通貨表示形式"),
+    ("Percent Style", "パーセント表示"),
+    ("Comma Style", "桁区切りスタイル"),
+    ("Increase Decimal", "小数点以下の桁数を増やす"),
+    ("Decrease Decimal", "小数点以下の桁数を減らす"),
+    ("More Accounting Formats…", "その他の通貨表示形式…"),
+    ("Conditional\nFormatting", "条件付き書式"),
+    ("Conditional Formatting", "条件付き書式"),
+    ("Format\nas Table", "テーブルとして書式設定"),
+    ("Cell\nStyles", "セルのスタイル"),
+    ("Format Cells…", "セルの書式設定…"),
+    ("Format Cell Alignment…", "セルの配置…"),
+    ("Format\nPane", "書式ウィンドウ"),
+    ("Format", "書式"),
+    ("Insert", "挿入"),
+    ("Insert Cells…", "セルの挿入…"),
+    ("Delete", "削除"),
+    ("Delete Cells…", "セルの削除…"),
+    ("Insert Sheet Rows", "シートの行を挿入"),
+    ("Insert Sheet Columns", "シートの列を挿入"),
+    ("Delete Sheet Rows", "シートの行を削除"),
+    ("Delete Sheet Columns", "シートの列を削除"),
+    ("Row Height…", "行の高さ…"),
+    ("AutoFit Row Height", "行の高さを自動調整"),
+    ("Column Width…", "列の幅…"),
+    ("AutoFit Column Width", "列の幅を自動調整"),
+    ("Default Width…", "標準の幅…"),
+    ("Hide Rows", "行を表示しない"),
+    ("Unhide Rows", "再表示"),
+    ("Hide Columns", "列の表示/非表示"),
+    ("Unhide Columns", "再表示"),
+    ("Lock Cell", "セルのロック"),
+    ("Clear", "クリア"),
+    ("Clear All", "すべてクリア"),
+    ("Clear Formats", "書式のクリア"),
+    ("Clear Contents", "数式と値のクリア"),
+    ("Clear Comments and Notes", "コメントとメモのクリア"),
+    ("Clear Hyperlinks", "ハイパーリンクのクリア"),
+    ("AutoSum", "オート SUM"),
+    ("AutoSum (⌘⇧T)", "オート SUM (⌘⇧T)"),
+    ("Flash\nFill", "フラッシュ フィル"),
+    ("Flash Fill", "フラッシュ フィル"),
+    ("Fill", "フィル"),
+    ("Clear Print Area", "印刷範囲のクリア"),
+    ("Find &\nSelect", "検索と選択"),
+    ("Find…", "検索…"),
+    ("Replace…", "置換…"),
+    ("Go To…", "ジャンプ…"),
+    ("Go To Special…", "セルを選択…"),
+    ("PivotTable", "ピボットテーブル"),
+    ("Recommended\nCharts", "推奨グラフ"),
+    ("Table", "テーブル"),
+    ("Pictures", "画像"),
+    ("Shapes", "図形"),
+    ("Icons", "アイコン"),
+    ("Text\nBox", "テキスト ボックス"),
+    ("Comment", "コメント"),
+    ("New\nComment", "新しいコメント"),
+    ("New Note", "新しいメモ"),
+    ("Show/Hide Note", "メモの表示/非表示"),
+    ("Header &\nFooter", "ヘッダーとフッター"),
+    ("Text to\nColumns", "区切り位置"),
+    ("Link", "リンク"),
+    ("Symbol", "記号"),
+    ("Sparklines", "スパークライン"),
+    ("Header & Footer", "ヘッダーとフッター"),
+    ("Draw with ink", "インクで描画"),
+    ("Eraser", "消しゴム"),
+    ("Ink to\nShape", "インクを図形に変換"),
+    ("Select\nObjects", "オブジェクトの選択"),
+    ("Themes", "テーマ"),
+    ("Margins", "余白"),
+    ("Size", "サイズ"),
+    ("Print\nArea", "印刷範囲"),
+    ("Set Print Area", "印刷範囲の設定"),
+    ("Breaks", "改ページ"),
+    ("Insert Page Break", "改ページの挿入"),
+    ("Remove Page Break", "改ページの解除"),
+    ("Reset All Page Breaks", "すべての改ページを解除"),
+    ("Print\nTitles", "印刷タイトル"),
+    ("Narrow", "狭い"),
+    ("Wide", "広い"),
+    ("Normal", "標準"),
+    ("Portrait", "縦"),
+    ("Landscape", "横"),
+    ("Custom Margins…", "ユーザー設定の余白…"),
+    ("Insert\nFunction", "関数の挿入"),
+    ("Name\nManager", "名前の管理"),
+    ("Define Name", "名前の定義"),
+    ("Use in Formula", "数式で使用"),
+    ("Create from Selection", "選択範囲から作成"),
+    ("Trace Precedents", "参照元のトレース"),
+    ("Trace Dependents", "参照先のトレース"),
+    ("Remove Arrows", "矢印の解除"),
+    ("Show Formulas", "数式の表示"),
+    ("Error Checking", "エラーチェック"),
+    ("Evaluate Formula", "数式の検証"),
+    ("Watch Window", "ウォッチ ウィンドウ"),
+    ("Calculation\nOptions", "計算方法の設定"),
+    ("Calculate Now", "再計算実行"),
+    ("Calculate Sheet", "シートの再計算"),
+    ("Automatic", "自動"),
+    ("Automatic Except for Data Tables", "データ テーブル以外は自動"),
+    ("Manual", "手動"),
+    ("Sort &\nFilter", "並べ替えとフィルター"),
+    ("Sort", "並べ替え"),
+    ("Sort A to Z", "昇順"),
+    ("Sort Z to A", "降順"),
+    ("Custom Sort…", "ユーザー設定の並べ替え…"),
+    ("Filter", "フィルター"),
+    ("Reapply", "再適用"),
+    ("Get Data\n(Text/CSV)", "データの取得 (テキスト/CSV)"),
+    ("Remove\nDuplicates", "重複の削除"),
+    ("Data\nValidation", "データの入力規則"),
+    ("Data Validation", "データの入力規則"),
+    ("Data Validation…", "データの入力規則…"),
+    ("Circle Invalid Data", "無効なデータをマーク"),
+    ("Clear Validation", "入力規則のクリア"),
+    ("Group", "グループ化"),
+    ("Ungroup", "グループ解除"),
+    ("Subtotal", "小計"),
+    ("What-If\nAnalysis", "What-If 分析"),
+    ("Data Table…", "データ テーブル…"),
+    ("Scenario Manager…", "シナリオ マネージャー…"),
+    ("Goal Seek…", "ゴール シーク…"),
+    ("Spelling", "スペルチェック"),
+    ("Check\nAccessibility", "アクセシビリティ チェック"),
+    ("Threaded Comments", "スレッド化されたコメント"),
+    ("Show\nComments", "コメントの表示"),
+    ("Protect\nWorkbook", "ブックの保護"),
+    ("Unprotect\nSheet", "シート保護の解除"),
+    ("Protect Sheet…", "シートの保護…"),
+    ("Comments", "コメント"),
+    ("Notes", "メモ"),
+    ("Page Break\nPreview", "改ページ プレビュー"),
+    ("Page\nLayout", "ページ レイアウト"),
+    ("Formula Bar", "数式バー"),
+    ("Gridlines", "目盛線"),
+    ("Headings", "見出し"),
+    ("Zoom", "ズーム"),
+    ("100%", "100%"),
+    ("Zoom to\nSelection", "選択範囲を拡大"),
+    ("Freeze\nPanes", "ウィンドウ枠の固定"),
+    ("Freeze Panes", "ウィンドウ枠の固定"),
+    ("Freeze Top Row", "最上行の固定"),
+    ("Freeze First Column", "最左列の固定"),
+    ("Unfreeze Panes", "ウィンドウ枠固定の解除"),
+    ("Dark Mode", "ダーク モード"),
+    ("Display theme", "表示テーマ"),
+    ("System", "システム"),
+    ("Light", "ライト"),
+    ("Dark", "ダーク"),
+    ("Interface language", "表示言語"),
+    ("Command\nPalette", "コマンド パレット"),
+    ("Agent\nControl", "エージェント制御"),
+    ("Action\nJournal", "操作ジャーナル"),
+    ("About\nGridCraft", "GridCraft について"),
+    ("Share", "共有"),
+    ("Add Chart\nElement", "グラフ要素を追加"),
+    ("Change\nChart Type", "グラフの種類の変更"),
+    ("Switch\nRow/Column", "行/列の切り替え"),
+    ("Data Labels: None", "データ ラベル: なし"),
+    ("Data Labels: Show", "データ ラベル: 表示"),
+    ("Legend: None", "凡例: なし"),
+    ("Legend: Bottom", "凡例: 下"),
+    ("Legend: Top", "凡例: 上"),
+    ("Legend: Right", "凡例: 右"),
+    ("Gridlines: None", "目盛線: なし"),
+    ("Gridlines: Show", "目盛線: 表示"),
+    ("Chart\nTitle", "グラフ タイトル"),
+    ("Delete\nChart", "グラフの削除"),
+    ("Convert\nto Range", "範囲に変換"),
+    ("Table\nStyles", "テーブル スタイル"),
+    ("Top 10 Items", "上位 10 項目"),
+    ("Top 10%", "上位 10%"),
+    ("Bottom 10 Items", "下位 10 項目"),
+    ("Bottom 10%", "下位 10%"),
+    ("Above Average", "平均より上"),
+    ("Below Average", "平均より下"),
+    ("Clear Rules from Entire Sheet", "シート全体からルールをクリア"),
+    ("Clear Rules from Selected Cells", "選択したセルからルールをクリア"),
+    ("Constants", "定数"),
+    ("Data Bars", "データ バー"),
+    ("Color Scales", "カラー スケール"),
+    ("Icon Sets", "アイコン セット"),
+    ("Home", "ホーム"),
+    ("Draw", "描画"),
+    ("Page Layout", "ページ レイアウト"),
+    ("Formulas", "数式"),
+    ("Data", "データ"),
+    ("Review", "レビュー"),
+    ("View", "表示"),
+    ("Automate", "自動化"),
+    ("Table Design", "テーブル デザイン"),
+    ("Chart Design", "グラフ デザイン"),
+    ("Ready", "準備完了"),
+    ("Enter", "入力"),
+    ("Edit", "編集"),
+    ("Point", "ポイント"),
+    ("Calculate", "計算"),
+    ("Select destination and press Enter or choose Paste", "貼り付け先を選び、Enter キーを押すか [貼り付け] を選びます"),
+    ("General", "標準"),
+    ("Number", "数値"),
+    ("Currency", "通貨"),
+    ("Accounting", "会計"),
+    ("Date", "日付"),
+    ("Time", "時刻"),
+    ("Percentage", "パーセンテージ"),
+    ("Fraction", "分数"),
+    ("Scientific", "指数"),
+    ("Text", "文字列"),
+    ("Special", "特殊"),
+    ("Custom", "ユーザー設定"),
+    ("Short Date", "日付"),
+    ("Long Date", "長い日付"),
+    ("More Number Formats…", "その他の表示形式…"),
+    ("Financial", "財務"),
+    ("Logical", "論理"),
+    ("Date &\nTime", "日付/時刻"),
+    ("Lookup &\nReference", "検索/行列"),
+    ("Math &\nTrig", "数学/三角"),
+    ("More\nFunctions", "その他の関数"),
+];
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn translations_are_unique_and_preserve_unknown_text() {
+        for (i, (en, ja)) in JAPANESE.iter().enumerate() {
+            assert!(!ja.is_empty());
+            assert!(JAPANESE.iter().take(i).all(|(other, _)| en != other));
+            assert_eq!(Language::En.tr(en), en.rsplit_once('|').map_or(*en, |(_, label)| label));
+        }
+        assert_eq!(Language::Ja.tr("Data"), "データ");
+        assert_eq!(Language::Ja.tr("Sheet1!A1"), "Sheet1!A1");
+        // A `context|label` key shows only its label in English and splits the translation.
+        assert_eq!(Language::En.tr("Page Layout|Orientation"), "Orientation");
+        assert_eq!(Language::Ja.tr("Page Layout|Orientation"), "印刷の向き");
+        assert_eq!(Language::Ja.tr("Orientation"), "方向");
+        assert_eq!(Language::parse("xx"), None);
+    }
+
+    #[test]
+    fn unknown_saved_language_keeps_the_other_preferences() {
+        let ui: crate::UiState = serde_json::from_str(r#"{"dark": true, "language": "xx"}"#).unwrap();
+        assert!(ui.dark);
+        assert_eq!(ui.language, Language::En);
+        let ui: crate::UiState = serde_json::from_str(r#"{"dark": true, "language": 7}"#).unwrap();
+        assert!(ui.dark);
+        let ui: crate::UiState = serde_json::from_str(r#"{"language": "ja"}"#).unwrap();
+        assert_eq!(ui.language, Language::Ja);
+        assert_eq!(crate::UiState::default().language, Language::En, "the default never reads the host locale");
+        assert_eq!(saved_language(&serde_json::json!({"language": "ja"})), Some(Language::Ja));
+        assert_eq!(saved_language(&serde_json::json!({"language": "xx"})), None);
+        assert_eq!(saved_language(&serde_json::json!({})), None);
+    }
+
+    #[test]
+    fn tags_reduce_to_a_supported_language() {
+        assert_eq!(Language::from_tag("ja-JP"), Some(Language::Ja));
+        assert_eq!(Language::from_tag("en_US"), Some(Language::En));
+        assert_eq!(Language::from_tag("de-DE"), None);
+    }
+
+    #[test]
+    fn language_commands_switch_and_persist() {
+        let mut app = crate::SheetApp::new(gridcraft_engine::Session::default(), Default::default());
+        app.run("app.language.japanese", serde_json::json!({})).unwrap();
+        assert_eq!(app.ui.language, Language::Ja);
+        let restored: crate::UiState = serde_json::from_str(&serde_json::to_string(&app.ui).unwrap()).unwrap();
+        assert_eq!(restored.language, Language::Ja);
+        app.run("app.language.english", serde_json::json!({})).unwrap();
+        assert_eq!(app.ui.language, Language::En);
+        app.run("app.language.set", serde_json::json!({"language": "ja"})).unwrap();
+        assert_eq!(app.ui.language, Language::Ja);
+        app.run("app.language.set", serde_json::json!({"code": "en"})).unwrap();
+        assert_eq!(app.ui.language, Language::En);
+        assert!(app.run("app.language.set", serde_json::json!({})).is_err(), "no language is an error, not a dialog");
+        for l in Language::ALL {
+            assert_eq!(Language::parse(l.code()), Some(l));
+        }
+    }
+}
diff --git a/crates/ui-egui/src/lib.rs b/crates/ui-egui/src/lib.rs
index c3efe3b..4422f1f 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -14,6 +14,7 @@ pub mod dialogs;
 pub mod editor;
 pub mod formula_bar;
 pub mod grid;
+pub mod i18n;
 pub mod icons;
 pub mod keytips;
 pub mod panes;
@@ -46,6 +47,12 @@ pub struct UiState {
     pub formula_bar_expanded: bool,
     pub status_bar: bool,
     pub recent: Vec<String>,
+    /// Interface language. English by default (so tests and headless renders never depend on the
+    /// host's locale); the desktop app starts from [`i18n::Language::system`] when `ui.json` has
+    /// no usable language (see [`i18n::saved_language`]). An unknown saved value reads as English
+    /// rather than failing the whole `UiState`.
+    #[serde(deserialize_with = "i18n::lenient")]
+    pub language: i18n::Language,
 }
 
 impl Default for UiState {
@@ -59,6 +66,7 @@ impl Default for UiState {
             formula_bar_expanded: false,
             status_bar: true,
             recent: vec![],
+            language: i18n::Language::En,
         }
     }
 }
@@ -278,6 +286,25 @@ impl SheetApp {
                 Ok(json!({"mode": self.ui.theme_mode()}))
             }
             "view.zoom100" => return Some(self.session.run("view.zoom", json!({"percent": 100})).inspect(|_| self.after_engine())),
+            "app.language.set" => {
+                // `{"language": "ja"}`; `code` is accepted as an alias.
+                let code = p.get("language").or_else(|| p.get("code")).and_then(Json::as_str).unwrap_or("");
+                match i18n::Language::parse(code) {
+                    Some(l) => {
+                        self.ui.language = l;
+                        Ok(json!({"language": l}))
+                    }
+                    None => Err(format!("unknown language {code:?} (use \"en\" or \"ja\")")),
+                }
+            }
+            "app.language.english" => {
+                self.ui.language = i18n::Language::En;
+                Ok(json!({"language": i18n::Language::En}))
+            }
+            "app.language.japanese" => {
+                self.ui.language = i18n::Language::Ja;
+                Ok(json!({"language": i18n::Language::Ja}))
+            }
             "ui.dialog" => {
                 let name = p.get("name").and_then(Json::as_str).unwrap_or("");
                 self.open_dialog(name, p.clone());
@@ -502,6 +529,8 @@ impl SheetApp {
         }
         let t0 = now_ms();
         text_box::before_ui(self, &ctx);
+        // The language the widgets translate with this frame (see `i18n::current`).
+        i18n::set_current(&ctx, self.ui.language);
         let t = theme::Tokens::get(&ctx);
         self.ribbon_keys(&ctx);
         ribbon::title_bar(self, ui);
diff --git a/crates/ui-egui/src/ribbon.rs b/crates/ui-egui/src/ribbon.rs
index 750d87d..5c0c8f5 100644
--- a/crates/ui-egui/src/ribbon.rs
+++ b/crates/ui-egui/src/ribbon.rs
@@ -140,7 +140,8 @@ pub fn show(app: &mut SheetApp, ui: &mut Ui) {
                         let active = app.ui.ribbon_tab == tab;
                         let contextual = ctx_tabs.contains(&tab);
                         let font = theme::ui_font(13.5);
-                        let tw = ui.painter().layout_no_wrap(tab.to_string(), font.clone(), t.text).size().x;
+                        let label = app.ui.language.tr(tab);
+                        let tw = ui.painter().layout_no_wrap(label.to_string(), font.clone(), t.text).size().x;
                         let (r, resp) = ui.allocate_exact_size(vec2(tw + 22.0, 30.0), Sense::click());
                         if resp.hovered() && !active {
                             ui.painter().rect_filled(r.shrink2(vec2(2.0, 4.0)), 5.0, t.hover);
@@ -152,7 +153,7 @@ pub fn show(app: &mut SheetApp, ui: &mut Ui) {
                         } else {
                             t.text_dim
                         };
-                        ui.painter().text(r.center(), Align2::CENTER_CENTER, tab, if active { theme::ui_bold(13.5) } else { font }, color);
+                        ui.painter().text(r.center(), Align2::CENTER_CENTER, label, if active { theme::ui_bold(13.5) } else { font }, color);
                         if active {
                             let ul = Rect::from_center_size(pos2(r.center().x, r.bottom() - 3.0), vec2(tw.clamp(20.0, 40.0), 3.0));
                             ui.painter().rect_filled(ul, 1.5, t.accent);
@@ -290,7 +291,8 @@ fn menu_items(app: &mut SheetApp, ui: &mut Ui, items: &[(&str, &str, serde_json:
             ui.separator();
             continue;
         }
-        let response = ui.add(egui::Button::new(*label).frame(false).min_size(vec2(220.0, 22.0)));
+        let lang = app.ui.language;
+        let response = ui.add(egui::Button::new(lang.tr(label)).frame(false).min_size(vec2(220.0, 22.0)));
         let sequence = match (app.keytips.prefix(), *id) {
             (Some("HV"), "dialog:pasteSpecial") => Some("HVS"),
             (Some("E"), "dialog:pasteSpecial") => Some("ES"),
@@ -518,7 +520,7 @@ fn home(app: &mut SheetApp, ui: &mut Ui) {
             if wrap.clicked() {
                 act(app, "home.wrapText", json!({}));
             }
-            ui.label(egui::RichText::new("Wrap Text").font(theme::ui_font(12.5)));
+            ui.label(egui::RichText::new(app.ui.language.tr("Wrap Text")).font(theme::ui_font(12.5)));
         });
         ui.horizontal(|ui| {
             for (icon, h, id, tip) in [
@@ -549,7 +551,7 @@ fn home(app: &mut SheetApp, ui: &mut Ui) {
             if m {
                 act(app, "home.mergeCenter", json!({}));
             }
-            ui.label(egui::RichText::new("Merge & Center").font(theme::ui_font(12.5)));
+            ui.label(egui::RichText::new(app.ui.language.tr("Merge & Center")).font(theme::ui_font(12.5)));
             egui::Popup::menu(&ma).show(|ui| {
                 menu_items(
                     app,
@@ -828,27 +830,28 @@ fn number_combo(app: &mut SheetApp, ui: &mut Ui, st: &Style) {
     let code = st.num_fmt.as_str().to_string();
     let kind = gridcraft_engine::display::number_format(&code).kind();
     let current = format!("{kind:?}");
+    let lang = app.ui.language;
     let sample = app.session.active().and_then(|d| d.wb.active().map(|sh| sh.value(d.selection.active))).unwrap_or_default();
     let mut picked: Option<&str> = None;
-    egui::ComboBox::from_id_salt("number_format").width(150.0).selected_text(egui::RichText::new(&current).font(theme::ui_font(12.5))).show_ui(
-        ui,
-        |ui| {
+    egui::ComboBox::from_id_salt("number_format")
+        .width(150.0)
+        .selected_text(egui::RichText::new(lang.tr(&current)).font(theme::ui_font(12.5)))
+        .show_ui(ui, |ui| {
             for name in
                 ["General", "Number", "Currency", "Accounting", "Short Date", "Long Date", "Time", "Percentage", "Fraction", "Scientific", "Text"]
             {
                 let code = gridcraft_engine::cmd::format::format_code_for(name);
                 let preview = app.session.active().map(|d| gridcraft_engine::display::format(&sample, code, &d.wb).text).unwrap_or_default();
-                let r = ui.add(egui::Button::selectable(current == name, format!("{name:<12}   {preview}")).min_size(vec2(240.0, 22.0)));
+                let r = ui.add(egui::Button::selectable(current == name, format!("{:<12}   {preview}", lang.tr(name))).min_size(vec2(240.0, 22.0)));
                 if r.clicked() {
                     picked = Some(name);
                 }
             }
             ui.separator();
-            if ui.button("More Number Formats…").clicked() {
+            if ui.button(lang.tr("More Number Formats…")).clicked() {
                 picked = Some("__more");
             }
-        },
-    );
+        });
     match picked {
         Some("__more") => app.open_dialog("formatCells", json!({"tab": "Number"})),
         Some(n) => act(app, "home.numberFormat", json!({"format": n})),
@@ -1307,7 +1310,7 @@ fn page_layout(app: &mut SheetApp, ui: &mut Ui) {
             ],
         );
     });
-    let o = big_button(ui, Icon::Orient, "Orientation", "Orientation", true);
+    let o = big_button(ui, Icon::Orient, "Page Layout|Orientation", "Page Layout|Orientation", true);
     egui::Popup::menu(&o).show(|ui| {
         menu_items(
             app,
@@ -1355,25 +1358,28 @@ fn page_layout(app: &mut SheetApp, ui: &mut Ui) {
         .active()
         .and_then(|d| d.wb.active().map(|s| (s.show_gridlines, s.show_headings, s.print.gridlines, s.print.headings)))
         .unwrap_or((true, true, false, false));
+    // These two checkboxes mean the screen view vs. the printed page, so they do not share the
+    // "View" tab label; pick per language rather than through the shared table.
+    let (screen, print) = if app.ui.language == crate::i18n::Language::Ja { ("画面", "印刷") } else { ("View", "Print") };
     ui.vertical(|ui| {
-        ui.label(egui::RichText::new("Gridlines").strong().small());
+        ui.label(egui::RichText::new(app.ui.language.tr("Gridlines")).strong().small());
         let mut v = sh.0;
-        if ui.checkbox(&mut v, "View").changed() {
+        if ui.checkbox(&mut v, screen).changed() {
             act(app, "view.gridlines", json!({"on": v}));
         }
         let mut p = sh.2;
-        if ui.checkbox(&mut p, "Print").changed() {
+        if ui.checkbox(&mut p, print).changed() {
             act(app, "pageLayout.printGridlines", json!({"on": p}));
         }
     });
     ui.vertical(|ui| {
-        ui.label(egui::RichText::new("Headings").strong().small());
+        ui.label(egui::RichText::new(app.ui.language.tr("Headings")).strong().small());
         let mut v = sh.1;
-        if ui.checkbox(&mut v, "View").changed() {
+        if ui.checkbox(&mut v, screen).changed() {
             act(app, "view.headings", json!({"on": v}));
         }
         let mut p = sh.3;
-        if ui.checkbox(&mut p, "Print").changed() {
+        if ui.checkbox(&mut p, print).changed() {
             act(app, "pageLayout.printHeadings", json!({"on": p}));
         }
     });
@@ -1618,17 +1624,18 @@ fn view(app: &mut SheetApp, ui: &mut Ui) {
     }
     sep(ui);
     let s = app.session.active().and_then(|d| d.wb.active().map(|s| (s.show_gridlines, s.show_headings))).unwrap_or((true, true));
+    let lang = app.ui.language;
     ui.vertical(|ui| {
         let mut fb = app.ui.formula_bar;
-        if ui.checkbox(&mut fb, "Formula Bar").changed() {
+        if ui.checkbox(&mut fb, lang.tr("Formula Bar")).changed() {
             app.ui.formula_bar = fb;
         }
         let mut g = s.0;
-        if ui.checkbox(&mut g, "Gridlines").changed() {
+        if ui.checkbox(&mut g, lang.tr("Gridlines")).changed() {
             act(app, "view.gridlines", json!({"on": g}));
         }
         let mut h = s.1;
-        if ui.checkbox(&mut h, "Headings").changed() {
+        if ui.checkbox(&mut h, lang.tr("Headings")).changed() {
             act(app, "view.headings", json!({"on": h}));
         }
     });
@@ -1658,21 +1665,37 @@ fn view(app: &mut SheetApp, ui: &mut Ui) {
         );
     });
     ui.vertical(|ui| {
-        ui.label("Display theme");
+        ui.label(lang.tr("Display theme"));
         let mode = app.ui.theme_mode();
         let label = match mode {
             "system" => "System",
             "dark" => "Dark",
             _ => "Light",
         };
-        egui::ComboBox::from_id_salt("display_theme").selected_text(label).width(88.0).show_ui(ui, |ui| {
+        egui::ComboBox::from_id_salt("display_theme").selected_text(lang.tr(label)).width(88.0).show_ui(ui, |ui| {
             for (value, label) in [("system", "System"), ("light", "Light"), ("dark", "Dark")] {
-                if ui.selectable_label(mode == value, label).clicked() {
+                if ui.selectable_label(mode == value, lang.tr(label)).clicked() {
                     act(app, "view.theme", json!({"mode": value}));
                 }
             }
         });
     });
+    sep(ui);
+    // Interface language: settings live in the View tab, like Excel's Options. The label follows
+    // the current language so it stays readable; the choices are always shown in their own script.
+    ui.vertical(|ui| {
+        ui.label(egui::RichText::new(lang.tr("Interface language")).font(theme::ui_font(11.5)).color(t.text_dim));
+        egui::ComboBox::from_id_salt("ui_language").width(120.0).selected_text(egui::RichText::new(lang.name()).font(theme::ui_font(12.5))).show_ui(
+            ui,
+            |ui| {
+                for l in crate::i18n::Language::ALL {
+                    if ui.selectable_label(l == lang, l.name()).clicked() {
+                        act(app, "app.language.set", json!({"language": l.code()}));
+                    }
+                }
+            },
+        );
+    });
 }
 
 fn automate(app: &mut SheetApp, ui: &mut Ui) {
diff --git a/crates/ui-egui/src/tabs.rs b/crates/ui-egui/src/tabs.rs
index 0258169..e4a4068 100644
--- a/crates/ui-egui/src/tabs.rs
+++ b/crates/ui-egui/src/tabs.rs
@@ -190,11 +190,11 @@ pub fn status_bar(app: &mut SheetApp, ui: &mut Ui) {
                     gridcraft_engine::Mode::Edit => "Edit",
                     gridcraft_engine::Mode::Point => "Point",
                 };
-                ui.label(egui::RichText::new(mode).font(theme::ui_font(12.5)).color(t.text_dim));
+                ui.label(egui::RichText::new(app.ui.language.tr(mode)).font(theme::ui_font(12.5)).color(t.text_dim));
                 if let Some(d) = app.session.active() {
                     if d.wb.calc.mode == gridcraft_engine::model::CalcMode::Manual {
                         ui.add_space(12.0);
-                        ui.label(egui::RichText::new("Calculate").font(theme::ui_font(12.5)).color(t.text_dim));
+                        ui.label(egui::RichText::new(app.ui.language.tr("Calculate")).font(theme::ui_font(12.5)).color(t.text_dim));
                     }
                     if !d.calc.circular.is_empty() {
                         ui.add_space(12.0);
@@ -205,7 +205,9 @@ pub fn status_bar(app: &mut SheetApp, ui: &mut Ui) {
                     if app.session.clipboard.is_some() && app.editor.is_none() {
                         ui.add_space(12.0);
                         ui.label(
-                            egui::RichText::new("Select destination and press Enter or choose Paste").font(theme::ui_font(12.5)).color(t.text_dim),
+                            egui::RichText::new(app.ui.language.tr("Select destination and press Enter or choose Paste"))
+                                .font(theme::ui_font(12.5))
+                                .color(t.text_dim),
                         );
                     }
                 }
diff --git a/crates/ui-egui/src/widgets.rs b/crates/ui-egui/src/widgets.rs
index f1b39fc..18649e6 100644
--- a/crates/ui-egui/src/widgets.rs
+++ b/crates/ui-egui/src/widgets.rs
@@ -4,9 +4,16 @@
 use egui::{Align2, Color32, Rect, Response, Sense, Stroke, StrokeKind, Ui, pos2, vec2};
 
 use crate::SheetApp;
+use crate::i18n;
 use crate::icons::{self, Icon};
 use crate::theme::{self, Tokens};
 
+/// Translates a label or tooltip for the language of the frame being drawn (English if untranslated).
+/// Ribbon chrome all flows through these widgets, so translating here localises it in one place.
+fn tr<'a>(ui: &Ui, text: &'a str) -> &'a str {
+    i18n::current(ui.ctx()).tr(text)
+}
+
 /// A flat square icon button with hover highlight and tooltip.
 pub fn icon_button(ui: &mut Ui, icon: Icon, color: Color32, tip: &str, size: egui::Vec2) -> Response {
     let (rect, resp) = ui.allocate_exact_size(size, Sense::click());
@@ -17,7 +24,7 @@ pub fn icon_button(ui: &mut Ui, icon: Icon, color: Color32, tip: &str, size: egu
         ui.painter().rect_filled(rect, 4.0, t.hover);
     }
     icons::paint(ui.painter(), Rect::from_center_size(rect.center(), vec2(16.0, 16.0)), icon, color);
-    resp.on_hover_text(tip)
+    resp.on_hover_text(tr(ui, tip))
 }
 
 /// A toggle-able small button (e.g. Bold) showing a checked state.
@@ -31,13 +38,14 @@ pub fn toggle_button(ui: &mut Ui, icon: Icon, on: bool, tip: &str) -> Response {
         ui.painter().rect_filled(rect, 4.0, t.hover);
     }
     icons::paint(ui.painter(), Rect::from_center_size(rect.center(), vec2(16.0, 16.0)), icon, t.text);
-    resp.on_hover_text(tip)
+    resp.on_hover_text(tr(ui, tip))
 }
 
 /// Large ribbon button: 32px icon over a one- or two-line label.
 pub fn big_button(ui: &mut Ui, icon: Icon, label: &str, tip: &str, dropdown: bool) -> Response {
     let t = Tokens::get(ui.ctx());
     let font = theme::ui_font(11.5);
+    let label = tr(ui, label);
     let lines: Vec<_> = label.split('\n').map(|line| ui.painter().layout_no_wrap(line.to_string(), font.clone(), t.text)).collect();
     let arrow_size = 10.0;
     let arrow_space = if dropdown { 4.0 + arrow_size } else { 0.0 };
@@ -65,13 +73,14 @@ pub fn big_button(ui: &mut Ui, icon: Icon, label: &str, tip: &str, dropdown: boo
             );
         }
     }
-    resp.on_hover_text(tip)
+    resp.on_hover_text(tr(ui, tip))
 }
 
 /// Small ribbon button: 16px icon with optional label to the right.
 pub fn small_button(ui: &mut Ui, icon: Icon, label: &str, tip: &str, dropdown: bool) -> Response {
     let t = Tokens::get(ui.ctx());
     let font = theme::ui_font(12.5);
+    let label = tr(ui, label);
     let tw = if label.is_empty() { 0.0 } else { ui.painter().layout_no_wrap(label.to_string(), font.clone(), t.text).size().x + 6.0 };
     let w = 24.0 + tw + if dropdown { 12.0 } else { 0.0 };
     let (rect, resp) = ui.allocate_exact_size(vec2(w, 23.0), Sense::click());
@@ -92,7 +101,7 @@ pub fn small_button(ui: &mut Ui, icon: Icon, label: &str, tip: &str, dropdown: b
             t.text_dim,
         );
     }
-    resp.on_hover_text(tip)
+    resp.on_hover_text(tr(ui, tip))
 }
 
 /// A split button: main part runs the default action, the arrow opens a menu.
@@ -113,7 +122,7 @@ pub fn split_button(ui: &mut Ui, icon: Icon, accent: Option<Color32>, tip: &str)
         ui.painter().rect_filled(Rect::from_min_size(pos2(main.left() + 4.0, main.bottom() - 5.0), vec2(17.0, 3.0)), 0.0, c);
     }
     icons::paint(ui.painter(), Rect::from_center_size(arrow.center(), vec2(10.0, 10.0)), Icon::Chevron, t.text_dim);
-    (m.on_hover_text(tip).clicked(), a)
+    (m.on_hover_text(tr(ui, tip)).clicked(), a)
 }
 
 /// Our colour palette (theme row + tints + standard colours), returns a picked hex colour,
diff --git a/crates/ui-egui/tests/interaction.rs b/crates/ui-egui/tests/interaction.rs
index 1cd0a8a..5baf10b 100644
--- a/crates/ui-egui/tests/interaction.rs
+++ b/crates/ui-egui/tests/interaction.rs
@@ -183,6 +183,32 @@ fn renders_every_ribbon_tab_without_panicking() {
     h.run_steps(3);
 }
 
+#[test]
+fn language_switch_translates_the_ribbon_and_persists() {
+    use gridcraft_ui_egui::i18n::Language;
+    let mut h = harness(blank());
+    // English by default whatever the host's locale: only the desktop app reads the system language.
+    assert_eq!(h.state().ui.language, Language::En);
+    // The language commands are listed with the other UI commands.
+    let ctx = h.ctx.clone();
+    let gridcraft_ui_egui::control::Outcome::Done(listed) = gridcraft_ui_egui::control::handle(h.state_mut(), &ctx, "engine.commands", &json!({}))
+    else {
+        panic!("engine.commands returns a response");
+    };
+    let ids: Vec<&str> = listed["result"].as_array().into_iter().flatten().filter_map(|c| c["id"].as_str()).collect();
+    for id in ["app.language.set", "app.language.english", "app.language.japanese"] {
+        assert!(ids.contains(&id), "engine.commands lists {id}");
+    }
+    h.state_mut().run("app.language.japanese", json!({})).unwrap();
+    h.run_steps(2); // the ribbon renders the Japanese tabs without panicking
+    assert_eq!(h.state().ui.language, Language::Ja);
+    // The preference round-trips through the saved UiState.
+    let restored: gridcraft_ui_egui::UiState = serde_json::from_str(&serde_json::to_string(&h.state().ui).unwrap()).unwrap();
+    assert_eq!(restored.language, Language::Ja);
+    h.state_mut().run("app.language.english", json!({})).unwrap();
+    assert_eq!(h.state().ui.language, Language::En);
+}
+
 #[test]
 fn dialogs_open_and_close() {
     let mut h = harness(blank());
diff --git a/crates/ui-egui/tests/system_theme.rs b/crates/ui-egui/tests/system_theme.rs
index 2a5365a..0d2cdea 100644
--- a/crates/ui-egui/tests/system_theme.rs
+++ b/crates/ui-egui/tests/system_theme.rs
@@ -44,7 +44,8 @@ fn set_mode(h: &mut egui_kittest::Harness<'static, SheetApp>, mode: &str) {
 }
 
 fn select_ribbon_theme(h: &mut egui_kittest::Harness<'static, SheetApp>, label: &str) {
-    h.get_by_role(egui::accesskit::Role::ComboBox).click();
+    // The View tab's first combo box is the display theme (the interface language follows it).
+    h.get_all_by_role(egui::accesskit::Role::ComboBox).next().expect("display theme combo").click();
     h.run_steps(2);
     h.get_by_role_and_label(egui::accesskit::Role::Button, label).click();
     h.run_steps(2);
diff --git a/docs/control-protocol.md b/docs/control-protocol.md
index 9c6c31b..e465a07 100644
--- a/docs/control-protocol.md
+++ b/docs/control-protocol.md
@@ -45,5 +45,11 @@ System follows live OS appearance changes and falls back to Light when the platf
 The saved choice remains System; new users still start in Light. The legacy
 `view.darkMode` command and `ui.set {dark}` select a fixed Light or Dark theme.
 
+`{command: "app.language.set", params: {language: "ja"}}` switches the interface language
+(`code` is accepted as an alias for `language`); `app.language.english` and
+`app.language.japanese` are shortcuts. Only interface chrome is translated, never command ids or
+document text. The choice is saved in `ui.json`; with nothing saved the desktop app follows the
+system language.
+
 For a headless equivalent (no window), use `gridcraft-cli mcp` or `gridcraft-cli run`;
 see [`mcp.md`](mcp.md) and [`cli.md`](cli.md).