add default culture when there's only 1 language configured

src/Umbraco.Web.BackOffice/Trees/ContentTreeController.cs

@@ -109,6 +109,11 @@ public class ContentTreeController : ContentTreeControllerBase, ISearchableTreeW
     {
         var culture = queryStrings?["culture"].ToString();
 
+        if(culture.IsNullOrWhiteSpace())
+        {
+            culture = _localizationService.GetDefaultLanguageIsoCode();
+        }
+
         IEnumerable<MenuItem> allowedUserOptions = GetAllowedUserMenuItemsForNode(entity);
         if (CanUserAccessNode(entity, allowedUserOptions, culture))
         {