Nav menu walker for multisite

This is my code:

public static function display_menu(){
        printf('<nav id="menu" class="column small-10">
                        %s
                </nav>', self::get_mobile_menu()
        );
}

public static function get_mobile_menu(){
    if( false === ( $mobile_menu = get_transient(self::MENUMOBILE_TRANSIENT_NAME) )){
        $mobile_menu_args = array(
                'theme_location'  => 'mobile',
                'echo'            => false,
                'container'       => '',
                'container_class' => '',
                'container_id'    => '',
                'menu_class'      => 'right',
                'items_wrap'      => '<ul class="%2$s">%3$s</ul>',
                'walker'          => new Mobile_Menu_Walker()
        );
        $mobile_menu = wp_nav_menu($mobile_menu_args);
        set_transient( self::MENUMOBILE_TRANSIENT_NAME, $mobile_menu, self::MENU_TRANSIENT_TIME );
    }
    return $mobile_menu;
}

If I switch to ENG version, the Walker nav create a menu with a div class="right" and ul without class… Why? in ITA version, it works!

The function "display_menu" is called into header.php