RouteConfig.cs

using System;
using System.Collections.Generic;
using System.Globalization;
using System.Linq;
using System.Threading;
using System.Web;
using System.Web.Mvc;
using System.Web.Routing;

namespace Pauwels.AspNet
{
    public class RouteConfig
    {
        public static void RegisterRoutes(RouteCollection routes)
        {
            routes.IgnoreRoute("{resource}.axd/{*pathInfo}");

            routes.MapRoute(
                "Localized",                                                              
                "{language}/{controller}/{action}/{id}",
                new { language = LocalizedRouteHandler.DefaultLanguage, controller = "Home", action = "Index", id = "" },
                constraints: new { language = LocalizedRouteHandler.LanguageConstraint }
            );

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

    class LocalizedRouteHandler : MvcRouteHandler
    {
        static string[] AvailableLanguages = "nl fr de".Split(' ');
        public static string DefaultLanguage = AvailableLanguages.First();

        public static string LanguageConstraint = String.Join("|",AvailableLanguages);

        static Dictionary<string, CultureInfo> Cultures =
            AvailableLanguages
                .Select(x => CultureInfo.CreateSpecificCulture(x))
                .ToDictionary(x => x.TwoLetterISOLanguageName);

        protected override IHttpHandler GetHttpHandler(RequestContext requestContext)
        {
            var currentLanguage = requestContext.RouteData.Values["language"].ToString().ToLower();
            var currentCulture = AvailableLanguages.Contains(currentLanguage) ? Cultures[currentLanguage] : Cultures[DefaultLanguage];
            Thread.CurrentThread.CurrentCulture = Thread.CurrentThread.CurrentUICulture = currentCulture;
            return base.GetHttpHandler(requestContext);
        }
    }
}