s1.cs

public class AutoLocalizingRoute : Route
{
    public AutoLocalizingRoute(string url, object defaults, object constraints)
        : base(url, new RouteValueDictionary(defaults), new RouteValueDictionary(constraints), new MvcRouteHandler()) { }
 
    public override VirtualPathData GetVirtualPath(RequestContext requestContext, RouteValueDictionary values)
    {
        // only set the culture if it's not present in the values dictionary yet
        // this check ensures that we can link to a specific language when we need to (fe: when picking your language)
        if (!values.ContainsKey("language"))
        {
            values["language"] = Thread.CurrentThread.CurrentCulture.TwoLetterISOLanguageName;
        }
 
        return base.GetVirtualPath(requestContext, values);
    }
}

s2.cs

var localizingRoute = new AutoLocalizingRoute("{language}/{controller}/{action}/{id}",
    new { id = UrlParameter.Optional }, new { language = "^[a-z]{2}$" });
RouteTable.Routes.Add("LocalizingRoute", localizingRoute);
 
RouteTable.Routes.MapRoute(
    "Default", // Route name
    "{controller}/{action}/{id}", // URL with parameters
    new { controller = "Home", action = "Index", id = UrlParameter.Optional } // Parameter defaults
);

s3.cs

private void OnBeginRequest(object sender, EventArgs e)
{
    var currentContext = new HttpContextWrapper(HttpContext.Current);
    var routeData = RouteTable.Routes.GetRouteData(currentContext);
    if (routeData == null || routeData.Values.Count == 0) return;
 
    if (routeData.Values["language"] == null)
    {
        RedirectToUrlWithAppropriateLanguage(currentContext, routeData);
    }
 
    var languageCode = (string)routeData.Values["language"];
    Thread.CurrentThread.CurrentUICulture = CultureInfo.GetCultureInfo(languageCode);
    Thread.CurrentThread.CurrentCulture = CultureInfo.CreateSpecificCulture(languageCode);
}