Fixes: U4-6307 Incorrect culture assigned to user (missing region code)

src/Umbraco.Core/Models/UserExtensions.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Globalization;
 using System.Linq;
+using System.Threading;
 using Umbraco.Core.Models.Membership;
 using Umbraco.Core.Services;
 
@@ -23,13 +24,21 @@ namespace Umbraco.Core.Models
 
         internal static CultureInfo GetUserCulture(string userLanguage, ILocalizedTextService textService)
         {
-            return textService.GetSupportedCultures()
-                .FirstOrDefault(culture =>
-                    //match on full name first
-                    culture.Name.InvariantEquals(userLanguage.Replace("_", "-")) ||
-                        //then match on the 2 letter name
-                    culture.TwoLetterISOLanguageName.InvariantEquals(userLanguage));
-        } 
+            try
+            {
+                var culture = CultureInfo.GetCultureInfo(userLanguage);
+                //TODO: This is a hack because we store the user language as 2 chars instead of the full culture 
+                // which is actually stored in the language files (which are also named with 2 chars!) so we need to attempt
+                // to convert to a supported full culture
+                var result = textService.ConvertToSupportedCultureWithRegionCode(culture);
+                return result;
+            }
+            catch (CultureNotFoundException)
+            {
+                //return the default one
+                return CultureInfo.GetCultureInfo("en");
+            }
+        }
 
         /// <summary>
         /// Checks if the user has access to the content item based on their start noe