<a href="/lang.php?lang=en">En</a>
lang.php:
$lang = $_GET['lang'];
$_SESSION['lang'] = $lang;
header('Location: /'.$lang);
index.php:
if(!isset($_SESSION['lang']) {
$_SESSION['lang'] = 'ru';
}
if(isset($_SESSION['lang']) && $_SESSION['lang'] == 'en') {
header('Location: /en');
}