Session["current_language"] = form["languageid"];
 return View();

public static class SessionFacade
{
  public static string CurrentLanguage
  {
    get
    {
      //Simply returns, but you could check for a null
      //and initialise it with a default value accordingly...
      return HttpContext.Current.Session["current_language"].ToString();
    }
    set
    {
      HttpContext.Current.Session["current_language"] = value;
    }
  }
}

public ActionResultChangelangue(FormCollection form)
{
  SessionFacade.CurrentLanguage = form["languageid"];
  return View();
}