Fixed umlauts not translating properly for username

src/Helpers/UsersHelper.cs

@@ -11,7 +11,7 @@ public static partial class UsersHelper
             throw new ArgumentException("Given name and surname must not be empty.");
 
         string combined = (surname + givenName).ToLowerInvariant();
-
+        combined = combined.Replace("ä", "ae").Replace("ö", "oe").Replace("ü", "ue");
         // Normalize to decompose accents (e.g., é -> e +  ́)
         string normalized = combined.Normalize(NormalizationForm.FormD);