core/gtk: add language config entry to override GUI localization (#10428)

Fixes #10276

<img width="853" height="637" alt="Screenshot From 2026-01-23 16-45-11"
src="https://github.com/user-attachments/assets/aff9d2f8-eb3e-411e-bd3d-ebd32e5c7973"
/>

src/apprt/gtk/class/application.zig

@@ -213,6 +213,11 @@ pub const Application = extern struct {
         /// Providers for loading custom stylesheets defined by user
         custom_css_providers: std.ArrayListUnmanaged(*gtk.CssProvider) = .empty,
 
+        /// A copy of the LANG environment variable that was provided to Ghostty
+        /// by the system. If this is null, the LANG environment variable did
+        /// not exist in Ghostty's environment variable.
+        saved_language: ?[:0]const u8 = null,
+
         pub var offset: c_int = 0;
     };
 
@@ -249,15 +254,6 @@ pub const Application = extern struct {
         gtk_version.logVersion();
         adw_version.logVersion();
 
-        // Set gettext global domain to be our app so that our unqualified
-        // translations map to our translations.
-        internal_os.i18n.initGlobalDomain() catch |err| {
-            // Failures shuldn't stop application startup. Our app may
-            // not translate correctly but it should still work. In the
-            // future we may want to add this to the GUI to show.
-            log.warn("i18n initialization failed error={}", .{err});
-        };
-
         // Load our configuration.
         var config = CoreConfig.load(alloc) catch |err| err: {
             // If we fail to load the configuration, then we should log
@@ -275,6 +271,27 @@ pub const Application = extern struct {
         };
         defer config.deinit();
 
+        const saved_language: ?[:0]const u8 = saved_language: {
+            const old_language = old_language: {
+                const result = (internal_os.getenv(alloc, "LANG") catch break :old_language null) orelse break :old_language null;
+                defer result.deinit(alloc);
+                break :old_language alloc.dupeZ(u8, result.value) catch break :old_language null;
+            };
+
+            if (config.language) |language| _ = internal_os.setenv("LANG", language);
+
+            break :saved_language old_language;
+        };
+
+        // Set gettext global domain to be our app so that our unqualified
+        // translations map to our translations.
+        internal_os.i18n.initGlobalDomain() catch |err| {
+            // Failures shuldn't stop application startup. Our app may
+            // not translate correctly but it should still work. In the
+            // future we may want to add this to the GUI to show.
+            log.warn("i18n initialization failed error={}", .{err});
+        };
+
         // Setup our GTK init env vars
         setGtkEnv(&config) catch |err| switch (err) {
             error.NoSpaceLeft => {
@@ -374,7 +391,7 @@ pub const Application = extern struct {
         // Setup our private state. More setup is done in the init
         // callback that GObject calls, but we can't pass this data through
         // to there (and we don't need it there directly) so this is here.
-        const priv = self.private();
+        const priv: *Private = self.private();
         priv.* = .{
             .rt_app = rt_app,
             .core_app = core_app,
@@ -383,6 +400,7 @@ pub const Application = extern struct {
             .css_provider = css_provider,
             .custom_css_providers = .empty,
             .global_shortcuts = gobject.ext.newInstance(GlobalShortcuts, .{}),
+            .saved_language = saved_language,
         };
 
         // Signals
@@ -415,11 +433,12 @@ pub const Application = extern struct {
     /// ensures that our memory is cleaned up properly.
     pub fn deinit(self: *Self) void {
         const alloc = self.allocator();
-        const priv = self.private();
+        const priv: *Private = self.private();
         priv.config.unref();
         priv.winproto.deinit(alloc);
         priv.global_shortcuts.unref();
         if (priv.transient_cgroup_base) |base| alloc.free(base);
+        if (priv.saved_language) |language| alloc.free(language);
         if (gdk.Display.getDefault()) |display| {
             gtk.StyleContext.removeProviderForDisplay(
                 display,
@@ -445,6 +464,12 @@ pub const Application = extern struct {
         return self.private().core_app.alloc;
     }
 
+    /// Get the original language that Ghostty was launched with. This returns a
+    /// pointer to internal memory so it must be copied by callers.
+    pub fn savedLanguage(self: *Self) ?[:0]const u8 {
+        return self.private().saved_language;
+    }
+
     /// Run the application. This is a replacement for `gio.Application.run`
     /// because we want more tight control over our event loop so we can
     /// integrate it with libghostty.

src/apprt/gtk/class/surface.zig

@@ -1595,10 +1595,17 @@ pub const Surface = extern struct {
     }
 
     pub fn defaultTermioEnv(self: *Self) !std.process.EnvMap {
-        const alloc = Application.default().allocator();
+        const app = Application.default();
+        const alloc = app.allocator();
         var env = try internal_os.getEnvMap(alloc);
         errdefer env.deinit();
 
+        if (app.savedLanguage()) |language| {
+            try env.put("LANG", language);
+        } else {
+            env.remove("LANG");
+        }
+
         // Don't leak these GTK environment variables to child processes.
         env.remove("GDK_DEBUG");
         env.remove("GDK_DISABLE");

src/config/Config.zig

@@ -94,6 +94,27 @@ pub const compatibility = std.StaticStringMap(
     .{ "macos-dock-drop-behavior", compatMacOSDockDropBehavior },
 });
 
+/// Set Ghostty's graphical user interface language to a language other than the
+/// system default language. The language must be fully specified, including the
+/// encoding. For example:
+///
+///     language = de_DE.UTF-8
+///
+/// will force the strings in Ghostty's graphical user interface to be in German
+/// rather than the system default.
+///
+/// This will not affect the language used by programs run _within_ Ghostty.
+/// Those will continue to use the default system language. There are also many
+/// non-GUI elements in Ghostty that are not translated - this setting will have
+/// no effect on those.
+///
+/// Warning: This setting cannot be reloaded at runtime. To change the language
+/// you must fully restart Ghostty.
+///
+/// GTK only.
+/// Available since 1.3.0.
+language: ?[:0]const u8 = null,
+
 /// The font families to use.
 ///
 /// You can generate the list of valid values using the CLI: