Improve localization structure and add missing translations

Thanks to @77155331 for Chinese translation (Issue #11)
Thanks to @klNuno for French localization (PR #13)

src/NebulaAuth/Core/LocalizationManager.cs

@@ -31,6 +31,8 @@ public static class LocManager
             LocalizationLanguage.English => "en",
             LocalizationLanguage.Russian => "ru",
             LocalizationLanguage.Ukrainian => "ua",
+            LocalizationLanguage.ChineseSimplified => "zh",
+            LocalizationLanguage.French => "fr",
             _ => throw new ArgumentOutOfRangeException(nameof(language), language, null)
         };
     }
@@ -107,5 +109,7 @@ public enum LocalizationLanguage
 {
     English,
     Russian,
-    Ukrainian
+    Ukrainian,
+    ChineseSimplified,
+    French
 }

src/NebulaAuth/Utility/LanguageUtility.cs

@@ -11,13 +11,16 @@ public static class LanguageUtility
     {
         var userCulture = CultureInfo.CurrentUICulture;
         var userLang = userCulture.TwoLetterISOLanguageName;
-        var userRegion = userCulture.Name;
+        var userRegion = new RegionInfo(userCulture.Name).TwoLetterISORegionName;
+
 
         switch (userLang)
         {
             case "ru": return LocalizationLanguage.Russian;
             case "uk": return LocalizationLanguage.Ukrainian;
             case "en": return LocalizationLanguage.English;
+            case "zh": return LocalizationLanguage.ChineseSimplified;
+            case "fr": return LocalizationLanguage.French;
         }
 
         if (userRegion.EndsWith("UA", StringComparison.OrdinalIgnoreCase))
@@ -28,9 +31,8 @@ public static class LanguageUtility
             "RU", "BY", "KZ", "KG", "TJ", "TM", "UZ", "AM", "AZ", "GE", "MD"
         ];
 
-        if (cisRegions.Any(r => userRegion.EndsWith(r, StringComparison.OrdinalIgnoreCase)))
-            return LocalizationLanguage.Russian;
-
-        return LocalizationLanguage.English;
+        return cisRegions.Any(r => userRegion.EndsWith(r, StringComparison.OrdinalIgnoreCase)) 
+            ? LocalizationLanguage.Russian 
+            : LocalizationLanguage.English;
     }
 }

src/NebulaAuth/ViewModel/Other/SettingsVM.cs

@@ -128,7 +128,9 @@ public partial class SettingsVM : ObservableObject
     {
         {LocalizationLanguage.English, "English"},
         {LocalizationLanguage.Russian, "Русский"},
-        {LocalizationLanguage.Ukrainian, "Українська"}
+        {LocalizationLanguage.Ukrainian, "Українська"},
+        {LocalizationLanguage.ChineseSimplified, "简体中文"},
+        {LocalizationLanguage.French, "Français"}
     };
 
     public Color? IconColor