Including a wp-lang file to define WPLANG according to user's choice (Wordpress)?

Question

In this previous questions, someone told me how to make WPLANG clickleable, so that the user can choose his preferred language (usually you can only define WPLANG by modifying the wp-config). But I'm sure how to make the link. Are the following functions giving me the possibility of using example/?lang= Or example/en?

*wp-lang.php

 session_start();
 if ( isset( $_GET['lang'] ) ) {
    $_SESSION['WPLANG'] = $_GET['lang'];
    define ('WPLANG', $_SESSION[WPLANG]);
 } else {
    if(isset($_SESSION['WPLANG'])) {
        define ('WPLANG', $_SESSION['WPLANG']);
        $_GET['lang'] = $_SESSION['WPLANG'];
    } else {
        if ( isset( $_SERVER["HTTP_ACCEPT_LANGUAGE"] ) ) {
            $languages = strtolower( $_SERVER["HTTP_ACCEPT_LANGUAGE"] );
             $languages = explode( ",", $languages );
            $_SESSION['WPLANG'] = $languages[0];
            $_SESSION['WPLANG'] = str_replace("-", "_", $_SESSION['WPLANG']);
            $_GET['lang'] = substr($_SESSION['WPLANG'],0,2);
            define ('WPLANG', $_SESSION[WPLANG]);
        } else {
            define ('WPLANG', '');
        }
    }
 }

*wp-config.php - Find the section where the constant WPLANG is defined. Add in the following line just before the WPLANG declaration.

 require_once(dirname(__FILE__).'/wp-lang.php');
 define ('WPLANG', '');