feat: persist desktop language preference

internal/api/app.go

@@ -1621,6 +1621,7 @@ func (a *App) GetAppSettings() map[string]interface{} {
 		"currentVaultPath": cfg.CurrentVaultPath,
 		"recentVaults":     cfg.RecentVaults,
 		"theme":            cfg.Theme,
+		"language":         cfg.Language,
 		"devMode":          cfg.DevMode,
 		"userPluginsDir":   cfg.UserPluginsDir,
 		"lastOpenedAt":     cfg.LastOpenedAt,
@@ -1634,18 +1635,33 @@ func (a *App) UpdateAppSettings(patch map[string]interface{}) string {
 	}
 
 	cfg := &appsettings.Config{}
+	hasConfigPatch := false
 	if v, ok := patch["theme"].(string); ok && v != "" {
 		cfg.Theme = v
+		hasConfigPatch = true
 	}
 	if v, ok := patch["devMode"].(bool); ok {
 		cfg.DevMode = v
+		hasConfigPatch = true
 	}
 	if v, ok := patch["userPluginsDir"].(string); ok && v != "" {
 		cfg.UserPluginsDir = v
+		hasConfigPatch = true
 	}
 
-	if err := a.appSettings.Update(cfg); err != nil {
-		return err.Error()
+	if hasConfigPatch {
+		if err := a.appSettings.Update(cfg); err != nil {
+			return err.Error()
+		}
+	}
+	if value, exists := patch["language"]; exists {
+		language, ok := value.(string)
+		if !ok {
+			return "language must be a string"
+		}
+		if err := a.appSettings.UpdateLanguage(language); err != nil {
+			return err.Error()
+		}
 	}
 	return ""
 }

internal/api/app_test.go

@@ -1294,6 +1294,34 @@ func assertSyncState(t *testing.T, service *syncsvc.Service, wantDeviceID, wantS
 	}
 }
 
+func TestAppSettingsLanguageAPI(t *testing.T) {
+	settings := appsettings.NewManager(filepath.Join(t.TempDir(), "config.json"))
+	if err := settings.Load(); err != nil {
+		t.Fatal(err)
+	}
+	if err := settings.Update(&appsettings.Config{Theme: "light", DevMode: true}); err != nil {
+		t.Fatal(err)
+	}
+	app := &App{appSettings: settings}
+
+	if got := app.GetAppSettings()["language"]; got != "system" {
+		t.Fatalf("initial language = %#v, want system", got)
+	}
+	if errStr := app.UpdateAppSettings(map[string]interface{}{"language": "ru"}); errStr != "" {
+		t.Fatalf("UpdateAppSettings language: %s", errStr)
+	}
+	got := app.GetAppSettings()
+	if got["language"] != "ru" || got["theme"] != "light" || got["devMode"] != true {
+		t.Fatalf("settings after language update = %#v", got)
+	}
+	if errStr := app.UpdateAppSettings(map[string]interface{}{"language": "de"}); errStr == "" {
+		t.Fatal("UpdateAppSettings accepted unsupported language")
+	}
+	if got := app.GetAppSettings()["language"]; got != "ru" {
+		t.Fatalf("language after rejected update = %#v, want ru", got)
+	}
+}
+
 func TestSetCurrentVaultInitializesWorkspaceWhenMissingAtStartup(t *testing.T) {
 	tmpDir := t.TempDir()
 	vaultParent := filepath.Join(tmpDir, "vault-parent")