diff --git a/apps/gridcraft/src/main.rs b/apps/gridcraft/src/main.rs
index 7343932..55dc9c2 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/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/i18n.rs b/crates/ui-egui/src/i18n.rs
index 48e809d..18fda08 100644
--- a/crates/ui-egui/src/i18n.rs
+++ b/crates/ui-egui/src/i18n.rs
@@ -28,6 +28,14 @@ impl Language {
         }
     }
 
+    /// 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),
@@ -56,16 +64,43 @@ impl Language {
         }
     }
 
+    /// 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
+        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 {
@@ -121,7 +156,8 @@ const JAPANESE: &[(&str, &str)] = &[
     ("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", "印刷の向き"),
+    ("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"),
@@ -153,7 +189,7 @@ const JAPANESE: &[(&str, &str)] = &[
     ("Column Width…", "列の幅…"),
     ("AutoFit Column Width", "列の幅を自動調整"),
     ("Default Width…", "標準の幅…"),
-    ("Hide Rows", "行の表示/非表示"),
+    ("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"),
@@ -218,7 +254,7 @@ const JAPANESE: &[(&str, &str)] = &[
     ("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", "依存元のトレース"),
+    ("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"),
@@ -274,6 +310,10 @@ const JAPANESE: &[(&str, &str)] = &[
     ("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"),
@@ -355,13 +395,32 @@ mod tests {
         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);
+            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));
@@ -378,5 +437,13 @@ mod tests {
         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 a5e4ab5..4458090 100644
--- a/crates/ui-egui/src/lib.rs
+++ b/crates/ui-egui/src/lib.rs
@@ -45,8 +45,11 @@ pub struct UiState {
     pub formula_bar_expanded: bool,
     pub status_bar: bool,
     pub recent: Vec<String>,
-    /// Interface language. Absent in older `ui.json` → the system's language (English fallback).
-    #[serde(default = "i18n::Language::system")]
+    /// 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,
 }
 
@@ -61,7 +64,7 @@ impl Default for UiState {
             formula_bar_expanded: false,
             status_bar: true,
             recent: vec![],
-            language: i18n::Language::system(),
+            language: i18n::Language::En,
         }
     }
 }
@@ -234,13 +237,14 @@ impl SheetApp {
             }
             "view.zoom100" => return Some(self.session.run("view.zoom", json!({"percent": 100})).inspect(|_| self.after_engine())),
             "app.language.set" => {
-                let code = p.get("language").and_then(Json::as_str).and_then(i18n::Language::parse);
-                match code {
+                // `{"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 {:?} (use \"en\" or \"ja\")", p.get("language").and_then(Json::as_str).unwrap_or(""))),
+                    None => Err(format!("unknown language {code:?} (use \"en\" or \"ja\")")),
                 }
             }
             "app.language.english" => {
diff --git a/crates/ui-egui/src/ribbon.rs b/crates/ui-egui/src/ribbon.rs
index 100bfc8..06bb02d 100644
--- a/crates/ui-egui/src/ribbon.rs
+++ b/crates/ui-egui/src/ribbon.rs
@@ -1246,7 +1246,7 @@ 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,
@@ -1626,8 +1626,7 @@ fn view(app: &mut SheetApp, ui: &mut Ui) {
             |ui| {
                 for l in crate::i18n::Language::ALL {
                     if ui.selectable_label(l == lang, l.name()).clicked() {
-                        let code = if l == crate::i18n::Language::Ja { "app.language.japanese" } else { "app.language.english" };
-                        act(app, code, json!({}));
+                        act(app, "app.language.set", json!({"language": l.code()}));
                     }
                 }
             },
diff --git a/crates/ui-egui/tests/interaction.rs b/crates/ui-egui/tests/interaction.rs
index ab33809..5baf10b 100644
--- a/crates/ui-egui/tests/interaction.rs
+++ b/crates/ui-egui/tests/interaction.rs
@@ -187,8 +187,18 @@ fn renders_every_ribbon_tab_without_panicking() {
 fn language_switch_translates_the_ribbon_and_persists() {
     use gridcraft_ui_egui::i18n::Language;
     let mut h = harness(blank());
-    // English by default on a machine whose system language is not Japanese.
-    assert_ne!(h.state().ui.language, Language::Ja);
+    // 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);
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).