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routes.MapRoute(
    name: "DefaultLocalized",
    url: "{lang}/{controller}/{action}/{id}",
    constraints: new { lang = @"(\w{2})|(\w{2}-\w{2})" },   // en or en-US
    defaults: new { controller = "Home", action = "Index", id = UrlParameter.Optional }
);

public class LocalizationAttribute : ActionFilterAttribute
{
    private string _DefaultLanguage = "en";
 
    public LocalizationAttribute(string defaultLanguage)
    {
        _DefaultLanguage = defaultLanguage;
    }
 
    public override void OnActionExecuting(ActionExecutingContext filterContext)
    {
        string lang = (string)filterContext.RouteData.Values["lang"] ?? _DefaultLanguage;
        if (lang != _DefaultLanguage)
        {
            try
            {
                Thread.CurrentThread.CurrentCulture =
                Thread.CurrentThread.CurrentUICulture = new CultureInfo(lang);
            }
            catch (Exception e)
            {
                throw new NotSupportedException(String.Format("ERROR: Invalid language code '{0}'.", lang));
            }
        }
    }
}