feat: add simple locale switch.

demo/Semi.Avalonia.Demo/Views/MainView.axaml

@@ -47,6 +47,21 @@
                         </Style>
                     </StackPanel.Styles>
                     
+                    <Button>
+                        <PathIcon Data="{StaticResource SemiIconLanguage}" />
+                        <Button.Flyout>
+                            <MenuFlyout Placement="Bottom" ItemsSource="{Binding LocaleItems}" />
+                        </Button.Flyout>
+                        <Button.Styles>
+                            <Style Selector="MenuItem" x:DataType="views:MenuItemViewModel">
+                                <Setter Property="Header" Value="{Binding Header}" />
+                                <Setter Property="ItemsSource" Value="{Binding Items}" />
+                                <Setter Property="Command" Value="{Binding Command}" />
+                                <Setter Property="CommandParameter" Value="{Binding CommandParameter}" />
+                            </Style>
+                        </Button.Styles>
+                    </Button>
+
                     <Button Command="{Binding OpenUrlCommand}" CommandParameter="{Binding DocumentationUrl}">
                         <PathIcon Data="{StaticResource SemiIconGlobe}" />
                     </Button>
@@ -60,7 +75,7 @@
                         Foreground="{DynamicResource ButtonDefaultTertiaryForeground}"
                         Command="{Binding ToggleThemeCommand}"
                         OnContent="{StaticResource SemiIconMoon}"
-                        OffContent="{StaticResource SemiIconSun}" />
+                        OffContent="{StaticResource SemiIconSun}"/>
 
                     <Button>
                         <PathIcon Data="{StaticResource SemiIconMenu}" />

demo/Semi.Avalonia.Demo/Views/MainView.axaml.cs

@@ -26,6 +26,8 @@ public partial class MainViewModel : ObservableObject
     public string RepoUrl => "https://github.com/irihitech/Semi.Avalonia";
     public IReadOnlyList<MenuItemViewModel> MenuItems { get; }
     
+    public IReadOnlyList<MenuItemViewModel> LocaleItems { get; }
+
     public MainViewModel()
     {
         MenuItems =
@@ -62,6 +64,52 @@ public partial class MainViewModel : ObservableObject
                 ]
             }
         ];
+        LocaleItems =
+        [
+            new MenuItemViewModel
+            {
+                Header = "Locale",
+                Items =
+                [
+                    new MenuItemViewModel
+                    {
+                        Header = "简体中文",
+                        Command = SelectLocaleCommand,
+                        CommandParameter = new System.Globalization.CultureInfo("zh-cn")
+                    },
+                    new MenuItemViewModel
+                    {
+                        Header = "English",
+                        Command = SelectLocaleCommand,
+                        CommandParameter = new System.Globalization.CultureInfo("en-us")
+                    },
+                    new MenuItemViewModel
+                    {
+                        Header = "日本語",
+                        Command = SelectLocaleCommand,
+                        CommandParameter = new System.Globalization.CultureInfo("ja-jp")
+                    },
+                    new MenuItemViewModel
+                    {
+                        Header = "Українська",
+                        Command = SelectLocaleCommand,
+                        CommandParameter = new System.Globalization.CultureInfo("uk-ua")
+                    },
+                    new MenuItemViewModel
+                    {
+                        Header = "Русский",
+                        Command = SelectLocaleCommand,
+                        CommandParameter = new System.Globalization.CultureInfo("ru-ru")
+                    },
+                    new MenuItemViewModel
+                    {
+                        Header = "繁體中文",
+                        Command = SelectLocaleCommand,
+                        CommandParameter = new System.Globalization.CultureInfo("zh-tw")
+                    },
+                ]
+            }
+        ];
     }
 
     [RelayCommand]
@@ -83,6 +131,16 @@ public partial class MainViewModel : ObservableObject
         }
     }
     
+    [RelayCommand]
+    private void SelectLocale(object? obj)
+    {
+        var app = Application.Current;
+        if (app is not null)
+        {
+            SemiTheme.OverrideLocaleResources(app, obj as System.Globalization.CultureInfo);
+        }
+    }
+
     [RelayCommand]
     private static async Task OpenUrl(string url)
     {

src/Semi.Avalonia/SemiTheme.axaml.cs

@@ -62,8 +62,9 @@ public class SemiTheme : Styles
         return _localeToResource[new CultureInfo("zh-cn")];
     }
 
-    public static void OverrideLocaleResources(Application application, CultureInfo culture)
+    public static void OverrideLocaleResources(Application application, CultureInfo? culture)
     {
+        if (culture is null) return;
         if (!_localeToResource.TryGetValue(culture, out var resources)) return;
         foreach (var kv in resources)
         {
@@ -71,8 +72,9 @@ public class SemiTheme : Styles
         }
     }
     
-    public static void OverrideLocaleResources(StyledElement element, CultureInfo culture)
+    public static void OverrideLocaleResources(StyledElement element, CultureInfo? culture)
     {
+        if (culture is null) return;
         if (!_localeToResource.TryGetValue(culture, out var resources)) return;
         foreach (var kv in resources)
         {