Url.Action not working with language parameter

I have only route. It is configured like :

        routes.MapRoute(
            name: "Default",
            url: "{lang}/{controller}/{action}/{id}",
            defaults: new { controller = "Home", action = "Index", lang = "en", id = UrlParameter.Optional },
            constraints: new  { lang=new LanguageConstraint()
            }
        );

I'm creating link using

 <a href='@Url.Action("Index", "Results", new {     lang=System.Threading.Thread.CurrentThread.CurrentCulture.TwoLetterISOLanguageName}')</a>

but link is not cerated at all. What I'm doing wrong? It seems fine. I have Index action and Results controller. It is ok when I have default route MVC comes with {controller}/{action}

This is LanguageConstraint:

      public class LanguageConstraint : IRouteConstraint
{
    public bool Match(System.Web.HttpContextBase httpContext, Route route, string parameterName, RouteValueDictionary values, RouteDirection routeDirection)
    {
        if (routeDirection == RouteDirection.IncomingRequest)
        {
            string language = values["lang"].ToString();  //(en|hr|de|it|fr|sk|nl|hu|sv|pl|cs|ru|sl
            if (language == "en" || language == "hr" || language == "de" || language == "it" || language == "fr" || language == "sk" || language == "nl" || 
                language == "hu" || language == "sv" || language == "pl" || language == "cs" || language == "ru" || language == "sl" )
                return true;
            else
                return false;
        }
        return false;
    }
}