MVC 5 How to define Owin LoginPath with localized routes

I exactly had the same problem and figured out a way to overcome this limitation.

In the CookieAuthenticationOptions options, there is a “Provider” property which is initialized with the CookieAuthenticationProvider. This implements a method called ApplyRedirect and a delegate OnApplyRedirect.
My first idea was to overwrite this ApplyRedirect and implement the required logic to handle localized routes. But unfortunately it can’t be overriden. Passing my logic to OnApplyRedirect causes to overwrite the default behavior. You theoretically can grab the source of this behavior, copy it into your project and modify it to your needs, but this is obviously not a good practice.
First, I decided to make a wrapper around the CookieAuthenticationProvider with two extension points using delegates and preserving the default behavior except of the url that is used – or easier, work around the wrapper (thx to lafi).

Then in the auth configuration I added my custom logic to the provider:

public void ConfigureAuth(IAppBuilder app)
{
    UrlHelper url = new UrlHelper(HttpContext.Current.Request.RequestContext);

    CookieAuthenticationProvider provider = new CookieAuthenticationProvider();

    var originalHandler = provider.OnApplyRedirect;

    //Our logic to dynamically modify the path (maybe needs some fine tuning)
    provider.OnApplyRedirect = context =>
    {
        var mvcContext = new HttpContextWrapper(HttpContext.Current);
        var routeData = RouteTable.Routes.GetRouteData(mvcContext);

        //Get the current language  
        RouteValueDictionary routeValues = new RouteValueDictionary();
        routeValues.Add("lang", routeData.Values["lang"]);

        //Reuse the RetrunUrl
        Uri uri = new Uri(context.RedirectUri);
        string returnUrl = HttpUtility.ParseQueryString(uri.Query)[context.Options.ReturnUrlParameter];
        routeValues.Add(context.Options.ReturnUrlParameter, returnUrl);

        //Overwrite the redirection uri
        context.RedirectUri = url.Action("login", "account", routeValues);
        originalHandler.Invoke(context);
    };

    app.UseCookieAuthentication(new CookieAuthenticationOptions
    {
        AuthenticationType = DefaultAuthenticationTypes.ApplicationCookie,
        LoginPath = new PathString(url.Action("login", "account")),
        //Set the Provider
        Provider = provider
    });
}

See also this code:

• CookieAuthenticationHandler
• CookieAuthenticationProvider

Hope it fits your needs.

UPDATE:
For less confusion, I updated my answer to use @Lafis enhancement, not using a wrapper class to apply the extended behavior. Please also give credit to @Lafis when upvoting.