var rm = new ResourceManager("ProductXYZ.Localization.App_GlobalResources.Messages", Assembly.Load("ProductXYZ.Localization"));
viewModel.Message = rm.GetString("SuccessfullySaveChanges", CultureInfo.CreateSpecificCulture(user.Language ?? CultureName));

protected void Application_PostAuthenticateRequest(object sender, EventArgs e)
{
    CultureInfo ci = CultureInfo.CreateSpecificCulture(HttpContext.Current.User.Language ?? CultureName);
    Thread t = System.Threading.Thread.CurrentThread;
    t.CurrentCulture = ci;
    t.CurrentUICulture = ci;
}

viewModel.Message = rm.GetString("SuccessfullySaveChanges", CultureInfo.CreateSpecificCulture(user.Language ?? CultureName));

viewModel.Message = rm.GetString("SuccessfullySaveChanges");