$valid_langs = array('en','fr','de','no');
if (isset($_GET['lang'])) {
  // verify that it is a valid language choice before attempting to use it
  // you may store an array of possibilities.

  if (in_array($_GET['lang'], $valid_langs)) {
    $language = $_GET['lang'];
    setcookie('lang', $_GET['lang']);
  }
}
else if (isset($_COOKIE['lang'])) {

  // also be sure to check that the language is a valid choice in a cookie too.
  if (in_array($_GET['lang'], $valid_langs)) {
    $language = $_COOKIE['lang'];
  }
}

$language = substr($_SERVER['HTTP_HOST'],0,2);