Update ModelStateExtensions.cs

Line 76
There is a duplicated word "errors" change this line 
/// Returns a list of cultures that have property validation errors errors
To
/// Returns a list of cultures that have property validation errors
Or if i missunderstood what you mean change with that for a better understanding.
/// Returns a list of cultures that have property validation errors "errors"

src/Umbraco.Web/ModelStateExtensions.cs

@@ -73,7 +73,7 @@ namespace Umbraco.Web
         }
 
         /// <summary>
-        /// Returns a list of cultures that have property validation errors errors
+        /// Returns a list of cultures that have property validation errors
         /// </summary>
         /// <param name="modelState"></param>
         /// <param name="localizationService"></param>