if(isset($_SESSION['lang'])){
$_SESSION['lang'] = 'ru';
}
include_once('lang'.$_SESSION['lang'].'.php');
<a href="/lang.php?lang=en">English</a>
<a href="/lang.php?lang=ru">Русский</a>
if(isset($_GET['lang'])){
$_SESSION['lang'] = $_GET['lang'];
}
header('location: /');