Keep a language code the starter cannot display in eu07.ini

Matches the old starter: lower case, and an unknown code is left alone.

StarterNG/App.axaml.cs

@@ -44,7 +44,18 @@ public partial class App : Avalonia.Application
             };
         AppServices.Current.Settings.Language = langName;
 
+        // What eu07.ini asked for, before the starter picks a language it can
+        // actually display.
+        string requestedCode = AppServices.Current.Settings.LanguageCode;
+
         ApplyLanguage(langName);
+
+        // The starter ships no translation for this code, so its own interface
+        // falls back - but the simulator has one and keeps it. Overwriting the key
+        // here would silently move eu07.exe to English on the next save.
+        if (!string.IsNullOrWhiteSpace(requestedCode) &&
+            !string.Equals(requestedCode, Loc.CurrentLangCode, StringComparison.OrdinalIgnoreCase))
+            AppServices.Current.Settings.LanguageCode = requestedCode;
     }
 
     public static void ApplyLanguage(string langNameOrCode)

StarterNG/Infrastructure/Settings/SettingsSerializer.cs

@@ -171,7 +171,10 @@ public sealed class SettingsSerializer
         foreach (string key in ObsoleteKeys)
             c.Remove(key);
 
-        c.Set("lang", string.IsNullOrWhiteSpace(s.LanguageCode) ? "en" : s.LanguageCode);
+        // Lower case, as the simulator expects and as the old starter wrote it.
+        c.Set("lang", string.IsNullOrWhiteSpace(s.LanguageCode)
+            ? "en"
+            : s.LanguageCode.Trim().ToLowerInvariant());
         c.SetBool("fullscreen", s.Fullscreen);
         c.SetBool("inactivepause", s.PauseWhenInactive);
         c.SetBool("pause", s.PauseOnStart);