WordPress: add rewrite rules

<?php // quick WordPress custom URL variables for /en/page-name & /fr/page-name

function flush_rewrite() {
    flush_rewrite_rules( false );
}
add_action( 'init', 'flush_rewrite' );

function my_insert_query_vars( $vars ) {
    array_push( $vars, 'lang' );
    return $vars;
}
add_filter( 'query_vars' , 'my_insert_query_vars' );

function add_rewrite_rules($aRules) {
    // $aNewRules = array('(en|fr)/([^/]+)/([^/]+)/?$' => 'index.php?posttype=$matches[2]&slug=$matches[3]&lang=$matches[1]');
    // $aNewRules = array('(en|fr)/project/([^/]+)/?$' => 'index.php?posttype=project&slug=$matches[3]&lang=$matches[1]');
    $aNewRules = array('(en|fr)/([^/]+)/?$' => 'index.php?pagename=$matches[2]&lang=$matches[1]');
    $aRules = $aNewRules + $aRules;
    return $aRules;
}
add_filter( 'rewrite_rules_array', 'add_rewrite_rules' );