WordPress Change Menus

functions.php :

add_theme_support( 'nav-menus' );
if( function_exists( 'register_nav_menus' ) ) {
    register_nav_menus(
        array(
            'english_menu' => 'English Menu',
            'french_menu' => 'French Menu'
        )
    );
}



header.php

<?php if( $post->post_parent == 8 ) : ?>
    <nav id="primary_menu">
        <?php
            wp_nav_menu(
                array(
                    'menu' => 'french_menu',
                    'container' => false,
                    'menu_class' => ''
                )
            );
        ?>
    </nav>
<?php elseif( $post->post_parent == 16 ) : ?>
    <nav id="primary_menu">
        <?php
            wp_nav_menu(
                array(
                    'menu' => 'english_menu',
                    'container' => false,
                    'menu_class' => ''
                )
            );
        ?>
    </nav>
<?php endif; ?>