diff --git a/src/Ursa.Themes.Semi/Index.axaml.cs b/src/Ursa.Themes.Semi/Index.axaml.cs
index a478b5aa..efa20383 100644
--- a/src/Ursa.Themes.Semi/Index.axaml.cs
+++ b/src/Ursa.Themes.Semi/Index.axaml.cs
@@ -19,7 +19,7 @@ public class SemiTheme : Styles
     };
 
     private static readonly ResourceDictionary _defaultResource = new zh_cn();
-    
+
     private CultureInfo? _locale;
 
     public SemiTheme(IServiceProvider? provider = null)
@@ -39,10 +39,16 @@ public CultureInfo? Locale
         {
             try
             {
-                _locale = value;
-                var resource = TryGetLocaleResource(value);
-                if (resource is null) return;
-                foreach (var kv in resource) Resources.Add(kv);
+                if (TryGetLocaleResource(value, out var resource) && resource is not null)
+                {
+                    _locale = value;
+                    foreach (var kv in resource) Resources[kv.Key] = kv.Value;
+                }
+                else
+                {
+                    _locale = new CultureInfo("zh-CN");
+                    foreach (var kv in _defaultResource) Resources[kv.Key] = kv.Value;
+                }
             }
             catch
             {
@@ -51,12 +57,28 @@ public CultureInfo? Locale
         }
     }
 
-    private static ResourceDictionary? TryGetLocaleResource(CultureInfo? locale)
+    private static bool TryGetLocaleResource(CultureInfo? locale, out ResourceDictionary? resourceDictionary)
     {
-        if (Equals(locale, CultureInfo.InvariantCulture)) return _defaultResource;
-        if (locale is null) return _localeToResource[new CultureInfo("zh-CN")];
-        if (_localeToResource.TryGetValue(locale, out var resource)) return resource;
-        return _localeToResource[new CultureInfo("zh-CN")];
+        if (Equals(locale, CultureInfo.InvariantCulture))
+        {
+            resourceDictionary = _defaultResource;
+            return true;
+        }
+
+        if (locale is null)
+        {
+            resourceDictionary = _defaultResource;
+            return false;
+        }
+
+        if (_localeToResource.TryGetValue(locale, out var resource))
+        {
+            resourceDictionary = resource;
+            return true;
+        }
+
+        resourceDictionary = _defaultResource;
+        return false;
     }
 
     public static void OverrideLocaleResources(Application application, CultureInfo? culture)
diff --git a/tests/HeadlessTest.Ursa/Semi/LocalizationTest.cs b/tests/HeadlessTest.Ursa/Semi/LocalizationTest.cs
new file mode 100644
index 00000000..fd6c0df4
--- /dev/null
+++ b/tests/HeadlessTest.Ursa/Semi/LocalizationTest.cs
@@ -0,0 +1,101 @@
+using System.Globalization;
+using Avalonia;
+using Avalonia.Controls;
+using Avalonia.Headless.XUnit;
+using Avalonia.Threading;
+using Avalonia.VisualTree;
+using Semi.Avalonia.Locale;
+using Ursa.Controls;
+using Ursa.Themes.Semi;
+
+namespace HeadlessTest.Ursa.Semi;
+
+public class LocalizationTest
+{
+    [AvaloniaFact]
+    public void Default_Locale_Is_Chinese()
+    {
+        var window = new UrsaWindow();
+        window.Show();
+        MessageBox.ShowOverlayAsync("Hello World", button: MessageBoxButton.YesNo, toplevelHashCode: window.GetHashCode());
+        Task.Delay(100).Wait();
+        Dispatcher.UIThread.RunJobs();
+        var dialog = window.GetVisualDescendants().OfType<MessageBoxControl>().SingleOrDefault();
+        var yesButton = dialog?.GetVisualDescendants().OfType<Button>().FirstOrDefault(b => b.Name == "PART_YesButton");
+        Assert.Equal("是", yesButton?.Content?.ToString());
+    }
+    
+    [AvaloniaFact]
+    public void Set_English_Works()
+    {
+        var window = new UrsaWindow();
+        window.Show();
+        MessageBox.ShowOverlayAsync("Hello World", button: MessageBoxButton.YesNo, toplevelHashCode: window.GetHashCode());
+        Task.Delay(100).Wait();
+        Dispatcher.UIThread.RunJobs();
+        var dialog = window.GetVisualDescendants().OfType<MessageBoxControl>().SingleOrDefault();
+        var yesButton = dialog?.GetVisualDescendants().OfType<Button>().FirstOrDefault(b => b.Name == "PART_YesButton");
+        Assert.Equal("是", yesButton?.Content?.ToString());
+        Assert.NotNull(Application.Current);
+        SemiTheme.OverrideLocaleResources(Application.Current, new CultureInfo("en-US"));
+        Assert.Equal("Yes", yesButton?.Content?.ToString());
+    }
+    
+    [AvaloniaFact]
+    public void Set_NonExisting_Culture_Does_Nothing()
+    {
+        var window = new UrsaWindow();
+        window.Show();
+        MessageBox.ShowOverlayAsync("Hello World", button: MessageBoxButton.YesNo, toplevelHashCode: window.GetHashCode());
+        Task.Delay(100).Wait();
+        Dispatcher.UIThread.RunJobs();
+        var dialog = window.GetVisualDescendants().OfType<MessageBoxControl>().SingleOrDefault();
+        var yesButton = dialog?.GetVisualDescendants().OfType<Button>().FirstOrDefault(b => b.Name == "PART_YesButton");
+        Assert.Equal("是", yesButton?.Content?.ToString());
+        Assert.NotNull(Application.Current);
+        // We expect there won't be anyone adding Armenian localization... Subject to change.
+        SemiTheme.OverrideLocaleResources(Application.Current, new CultureInfo("hy-AM"));
+        Assert.Equal("是", yesButton?.Content?.ToString());
+        SemiTheme.OverrideLocaleResources(Application.Current, null);
+        Assert.Equal("是", yesButton?.Content?.ToString());
+    }
+    
+    [AvaloniaFact]
+    public void Set_English_To_Control_Works()
+    {
+        var window = new UrsaWindow();
+        window.Show();
+        MessageBox.ShowOverlayAsync("Hello World", button: MessageBoxButton.YesNo, toplevelHashCode: window.GetHashCode());
+        Task.Delay(100).Wait();
+        Dispatcher.UIThread.RunJobs();
+        var dialog = window.GetVisualDescendants().OfType<MessageBoxControl>().SingleOrDefault();
+        var yesButton = dialog?.GetVisualDescendants().OfType<Button>().FirstOrDefault(b => b.Name == "PART_YesButton");
+        Assert.Equal("是", yesButton?.Content?.ToString());
+        Assert.NotNull(Application.Current);
+        SemiTheme.OverrideLocaleResources(window, new CultureInfo("en-US"));
+        Assert.Equal("Yes", yesButton?.Content?.ToString());
+    }
+    
+    [Fact]
+    public void SemiTheme_Localization_Behavior()
+    {
+        var theme = new SemiTheme();
+        Assert.Null(theme.Locale);
+        theme.Locale = new CultureInfo("en-US");
+        Assert.Equal(new CultureInfo("en-US"), theme.Locale);
+        var yesText = theme.Resources["STRING_MENU_DIALOG_YES"];
+        Assert.Equal("Yes", yesText);
+        theme.Locale = new CultureInfo("zh-CN");
+        Assert.Equal(new CultureInfo("zh-CN"), theme.Locale);
+        yesText = theme.Resources["STRING_MENU_DIALOG_YES"];
+        Assert.Equal("是", yesText);
+        theme.Locale = new CultureInfo("hy-AM");
+        Assert.Equal(new CultureInfo("zh-CN"), theme.Locale);
+        yesText = theme.Resources["STRING_MENU_DIALOG_YES"];
+        Assert.Equal("是", yesText);
+        theme.Locale = null;
+        Assert.Equal(new CultureInfo("zh-CN"), theme.Locale);
+        yesText = theme.Resources["STRING_MENU_DIALOG_YES"];
+        Assert.Equal("是", yesText);
+    }
+}
\ No newline at end of file