I’m trying to clean the URL in wordpress.
instead of this www.website.com/mypage/?myvariable=fr
I want to have this www.website.com/fr/mypage/

The redirection works well, but I can not get the value I put in the URL as a $_GET value.

In the first part I used add_rewrite_rule seems to work well. The redirection is OK.

function add_rewrite(){
    flush_rewrite_rules();
    add_rewrite_rule(
        'fr/([a-z0-9-]+)',
        'index.php?pagename=$matches[1]&myvariable=fr',
        'top'
    );
}
add_action('init', 'add_rewrite');

But I can not get the value of myvariable wich looks empty

echo get_query_var("myvariable");

even if I use the folowing code

function add_query_vars($aVars) {
    $aVars[] = "myvariable";
    return $aVars;
}
add_filter('query_vars', 'add_query_vars');