void protected OnLogin()
{
    if(UserIsAuthenticated)
    {
        Session["English"] = true;
    }
}

void Application_BeginRequest(object sender, EventArgs e)
{
    if(Request.PhysicalPath.Contains("English")
    {
         if(!((bool)Session["English"]))
             //Not "English" user - redirect to login or unauthorized page
    }
}