Asp.Net MVC 3 variable in url

To meet your needs change the Route config to:

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

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

The two key parts are the route itself, {language}/{controller}/{action}/{id} and the constraint part, new {language=new LanguageConstraint()}.

The first part will select the {language} part as a variable (default being en for now) to the controller. The controller signature:

 public ActionResult Index(string language) {

will pick up the new language variable. Since adding language to each and every controller could seem cumbersome you could create a ViewModelBase class to passed to every controller with a property that contains the language value, which every subsequent View Model class inherits from.

Without a constraint the route pattern would pick up all values in the url for the language part and writing a Regex expression to match all wanted language values would be tedious, I think it's easier to write an IRouteConstraint based class similar to the following:

public class LanguageConstraint : IRouteConstraint{
    public bool Match(HttpContextBase httpContext, Route route, string parameterName, RouteValueDictionary values,
                      RouteDirection routeDirection) {

        //create accepted lanaguages collection from somewhere.
        string[] languageArray = new[]{"en","jp", "de"};

        string language = values["language"].ToString();

        if (string.IsNullOrEmpty(language))
            return false;

        return languageArray.FirstOrDefault(l=>l.Equals(language,StringComparison.InvariantCultureIgnoreCase)) != null;
    }
}

Simply it creates a list of known language values and check the provided language value against that list. If it doesn't exist false is returned and a 404 is thrown.