feat(settings-ui): crash fix - add back the missing default language option to resources (#45766)

Add back the default language option to the resources for better
localization support. This change is fixing a regression:
 Two bugs here:
1. The original commit by Stefan had a case mismatch (Default_language
vs Default_Language) — latent bug
2. Niels's PR used Copilot CLI to find dead strings, which incorrectly
removed a live resource due to the case
  mismatch

---------

Co-authored-by: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/settings-ui/Settings.UI/Strings/en-us/Resources.resw

@@ -4772,6 +4772,9 @@ The break timer font matches the text font.</value>
   <data name="LanguageRestartInfoButton.Content" xml:space="preserve">
     <value>Restart</value>
   </data>
+  <data name="Default_Language" xml:space="preserve">
+    <value>Windows default</value>
+  </data>
   <data name="Arabic_Saudi_Arabia_Language" xml:space="preserve">
     <value>Arabic (Saudi Arabia)</value>
   </data>

src/settings-ui/Settings.UI/ViewModels/GeneralViewModel.cs

@@ -226,7 +226,7 @@ namespace Microsoft.PowerToys.Settings.UI.ViewModels
         // Supported languages. Taken from Resources.wxs + default + en-US
         private Dictionary<string, string> langTagsAndIds = new Dictionary<string, string>
         {
-            { string.Empty, "Default_language" },
+            { string.Empty, "Default_Language" },
             { "ar-SA", "Arabic_Saudi_Arabia_Language" },
             { "cs-CZ", "Czech_Language" },
             { "de-DE", "German_Language" },