diff --git a/Cargo.lock b/Cargo.lock
index 6af8dbc2..56413f89 100644
--- a/Cargo.lock
+++ b/Cargo.lock
@@ -1624,6 +1624,7 @@ dependencies = [
  "rayon",
  "serde",
  "serde_json",
+ "sys-locale",
  "toml",
  "web-time",
 ]
@@ -4949,6 +4950,15 @@ dependencies = [
  "syn 3.0.6",
 ]
 
+[[package]]
+name = "sys-locale"
+version = "0.3.2"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "8eab9a99a024a169fe8a903cf9d4a3b3601109bcc13bd9e3c6fff259138626c4"
+dependencies = [
+ "libc",
+]
+
 [[package]]
 name = "tag_ptr"
 version = "0.1.0"
diff --git a/Cargo.toml b/Cargo.toml
index c35c6738..2c6ffc83 100644
--- a/Cargo.toml
+++ b/Cargo.toml
@@ -103,6 +103,8 @@ eframe = { version = "0.36", default-features = false }
 egui_kittest = "0.36"
 rfd = "0.17"
 cpal = "0.16"
+# The operating system's interface language (MIT/Apache-2.0, no dependencies on desktop).
+sys-locale = "0.3"
 
 # Local co-development against a FilmCraft checkout next to this repo: uncomment.
 # [patch."https://github.com/storytold/filmcraft"]
diff --git a/crates/engine/src/prefs.rs b/crates/engine/src/prefs.rs
index a0d305e9..376d0d98 100644
--- a/crates/engine/src/prefs.rs
+++ b/crates/engine/src/prefs.rs
@@ -22,6 +22,13 @@ pub const PREFS_VERSION: u32 = 2;
 /// Settings file name in the config store.
 pub const PREFS_FILE: &str = "prefs.json";
 
+/// Settings ▸ General ▸ Language: (label, `general.language` value).
+pub const LANGUAGES: &[(&str, &str)] = &[("Match System", "system"), ("English", "en"), ("日本語", "ja")];
+
+fn is_language(v: &str) -> bool {
+    LANGUAGES.iter().any(|(_, l)| *l == v)
+}
+
 /// Unknown keys inside a page, kept so a newer version's settings survive a round trip.
 type Extra = BTreeMap<String, Value>;
 
@@ -44,8 +51,9 @@ macro_rules! page {
 }
 
 page!(General {
-    /// Interface language (`en` or `ja`).
-    language: String = "en".into(),
+    /// Interface language: `system` (the operating system's, where EffectCraft has it, else
+    /// English), `en` or `ja`.
+    language: String = "system".into(),
     /// Levels of Undo (1–99).
     undo_levels: u32 = 32,
     /// Path Point and Handle Size (px).
@@ -417,8 +425,8 @@ impl Prefs {
     pub fn normalize(&mut self) {
         self.version = PREFS_VERSION;
         let g = &mut self.general;
-        if !matches!(g.language.as_str(), "en" | "ja") {
-            g.language = "en".into();
+        if !is_language(&g.language) {
+            g.language = General::default().language;
         }
         g.undo_levels = g.undo_levels.clamp(1, 99);
         g.path_point_size = g.path_point_size.clamp(3, 20);
@@ -458,8 +466,9 @@ impl Prefs {
 
     /// Set the value at a dotted key. The key must exist and the value must have its type.
     pub fn set(&mut self, key: &str, value: Value) -> Result<(), String> {
-        if key == "general.language" && !matches!(value.as_str(), Some("en" | "ja")) {
-            return Err("`general.language` expects `en` or `ja`".into());
+        if key == "general.language" && !value.as_str().is_some_and(is_language) {
+            let all: Vec<&str> = LANGUAGES.iter().map(|(_, l)| *l).collect();
+            return Err(format!("`general.language` expects one of {}", all.join(", ")));
         }
         let mut v = serde_json::to_value(&*self).map_err(|e| e.to_string())?;
         let ptr = format!("/{}", key.replace('.', "/"));
@@ -885,7 +894,7 @@ pub fn pages() -> Vec<Page> {
             id: "general",
             title: "General",
             items: vec![
-                s("general.language", "Language", Kind::Choice(&[("English", "en"), ("日本語", "ja")]), true),
+                s("general.language", "Language", Kind::Choice(LANGUAGES), true),
                 s("general.undoLevels", "Levels of Undo", Kind::Int(1, 99, ""), true),
                 s("general.pathPointSize", "Path Point and Handle Size", Kind::Int(3, 20, "px"), true),
                 s("general.recentItems", "Recent Projects Shown", Kind::Int(1, 30, ""), true),
diff --git a/crates/engine/src/tests_settings.rs b/crates/engine/src/tests_settings.rs
index 4afa08fc..dc3d9888 100644
--- a/crates/engine/src/tests_settings.rs
+++ b/crates/engine/src/tests_settings.rs
@@ -491,7 +491,8 @@ fn memory_tick_never_waits_for_the_system() {
 fn interface_language_is_validated_persisted_and_backward_compatible() {
     let store = Arc::new(MemoryConfig::default());
     let mut s = Session { config: Some(store.clone()), ..Default::default() };
-    assert_eq!(s.prefs.general.language, "en");
+    assert_eq!(s.prefs.general.language, "system", "Match System by default (#229)");
+    s.execute("prefs.set", json!({"key": "general.language", "value": "en"})).unwrap();
     s.execute("prefs.set", json!({"key": "general.language", "value": "ja"})).unwrap();
     assert_eq!(s.execute("prefs.get", json!({"key": "general.language"})).unwrap(), json!("ja"));
     let saved = store.read(PREFS_FILE).unwrap();
@@ -504,9 +505,10 @@ fn interface_language_is_validated_persisted_and_backward_compatible() {
     reloaded.load_settings();
     assert_eq!(reloaded.prefs.general.language, "ja");
     reloaded.execute("prefs.reset", json!({"page": "general"})).unwrap();
-    assert_eq!(reloaded.prefs.general.language, "en");
-    assert_eq!(Prefs::from_json(r#"{"general":{"undoLevels":17}}"#).general.language, "en");
-    assert_eq!(Prefs::from_json(r#"{"general":{"language":"unknown"}}"#).general.language, "en");
+    assert_eq!(reloaded.prefs.general.language, "system");
+    assert_eq!(Prefs::from_json(r#"{"general":{"undoLevels":17}}"#).general.language, "system");
+    assert_eq!(Prefs::from_json(r#"{"general":{"language":"unknown"}}"#).general.language, "system");
+    assert_eq!(Prefs::from_json(r#"{"general":{"language":"en"}}"#).general.language, "en", "a chosen language stays");
 }
 
 #[test]
diff --git a/crates/ui-egui/Cargo.toml b/crates/ui-egui/Cargo.toml
index 5bdfbf4f..2d07038d 100644
--- a/crates/ui-egui/Cargo.toml
+++ b/crates/ui-egui/Cargo.toml
@@ -30,6 +30,9 @@ web-time = { workspace = true }
 kurbo = { workspace = true }
 image = { workspace = true }
 
+[target.'cfg(not(target_arch = "wasm32"))'.dependencies]
+sys-locale = { workspace = true }
+
 [dev-dependencies]
 egui_kittest = { workspace = true, features = ["eframe", "wgpu"] }
 pollster = "0.4"
diff --git a/crates/ui-egui/src/i18n.rs b/crates/ui-egui/src/i18n.rs
index bdbeafb3..f864b064 100644
--- a/crates/ui-egui/src/i18n.rs
+++ b/crates/ui-egui/src/i18n.rs
@@ -7,7 +7,31 @@ use crate::EffectcraftApp;
 use effectcraft_engine::menus::MenuEntry;
 
 pub(crate) fn japanese(app: &EffectcraftApp) -> bool {
-    app.session.prefs.general.language == "ja"
+    match app.session.prefs.general.language.as_str() {
+        "system" => system_language() == "ja",
+        l => l == "ja",
+    }
+}
+
+/// Settings ▸ General ▸ Language ▸ Match System: the operating system's interface language
+/// where EffectCraft has it, else English (#229), as After Effects installs in the system's
+/// language. The browser build stays in English: it has no Japanese font of its own.
+fn system_language() -> &'static str {
+    #[cfg(not(target_arch = "wasm32"))]
+    {
+        static LANGUAGE: std::sync::OnceLock<&'static str> = std::sync::OnceLock::new();
+        LANGUAGE.get_or_init(|| supported(sys_locale::get_locale().as_deref()))
+    }
+    #[cfg(target_arch = "wasm32")]
+    "en"
+}
+
+/// The language EffectCraft shows for a BCP 47 locale (`ja-JP` → `ja`).
+fn supported(locale: Option<&str>) -> &'static str {
+    match locale.and_then(|l| l.split(['-', '_']).next()) {
+        Some(l) if l.eq_ignore_ascii_case("ja") => "ja",
+        _ => "en",
+    }
 }
 
 pub(crate) fn label<'a>(app: &EffectcraftApp, command: &str, source: &'a str) -> &'a str {
@@ -723,6 +747,7 @@ mod tests {
     #[test]
     fn language_changes_native_labels_without_changing_commands_or_parameters() {
         let mut app = EffectcraftApp::new(effectcraft_engine::Session::default());
+        app.session.execute("prefs.set", json!({"key":"general.language", "value":"en"})).unwrap();
         let en = crate::native_menu::build(&app);
         let state = crate::native_menu::state_key(&app);
         app.session.execute("prefs.set", json!({"key":"general.language", "value":"ja"})).unwrap();
@@ -749,6 +774,26 @@ mod tests {
         app.session.execute("prefs.set", json!({"key":"general.language", "value":"en"})).unwrap();
         assert_eq!(label(&app, "", "Composition"), "Composition");
     }
+    #[test]
+    fn match_system_uses_the_system_language_where_there_is_a_catalog() {
+        for (locale, language) in [
+            (Some("ja-JP"), "ja"),
+            (Some("ja"), "ja"),
+            (Some("JA_jp.UTF-8"), "ja"),
+            (Some("en-US"), "en"),
+            (Some("jv-ID"), "en"),
+            (Some("de-DE"), "en"),
+            (None, "en"),
+        ] {
+            assert_eq!(supported(locale), language, "{locale:?}");
+        }
+        let mut app = EffectcraftApp::new(effectcraft_engine::Session::default());
+        assert_eq!(app.session.prefs.general.language, "system", "the default");
+        assert_eq!(japanese(&app), system_language() == "ja");
+        app.session.execute("prefs.set", json!({"key":"general.language", "value":"ja"})).unwrap();
+        assert!(japanese(&app));
+    }
+
     #[test]
     fn general_settings_exposes_the_language_automation_id() {
         let mut app = EffectcraftApp::new(effectcraft_engine::Session::default());
diff --git a/docs/preferences.md b/docs/preferences.md
index ee1fe80c..b20ee8b7 100644
--- a/docs/preferences.md
+++ b/docs/preferences.md
@@ -29,7 +29,7 @@ Commands (CLI, MCP, control channel):
 
 ### Settings that change behaviour
 
-- `general.language`: interface language (`en` / `ja`), Settings ▸ General ▸ Language; menu labels change immediately. Native Japanese UI uses installed system fonts; the web host must supply a Japanese font.
+- `general.language`: interface language (`system` / `en` / `ja`), Settings ▸ General ▸ Language; menu labels change immediately. `system` (Match System, the default) follows the operating system's interface language where EffectCraft has a translation and is English otherwise; the browser build stays in English with it. Native Japanese UI uses installed system fonts; the web host must supply a Japanese font.
 - `general.undoLevels`: Levels of Undo
 - `general.pathPointSize`: Path Point and Handle Size
 - `general.recentItems`: Recent Projects Shown