Nav Walker – custom sub-menu class

1st level sub menu

<ul class="sub-menu-1">

2nd level sub menu

<ul class="sub-menu-2">

<?php

class Custom_Walker_Nav_Menu extends Walker_Nav_Menu {

    function start_lvl(&$output, $depth = 0, $args = array()) {
        $indent = str_repeat("\t", $depth);
        $submenu_class = 'sub-menu-' . ($depth + 1); 
        $output .= "\n$indent<ul class=\"$submenu_class\">\n";
    }
}

wp_nav_menu(array(
    'theme_location' => 'desktop_menu',
    'menu_class'     => 'desktop-menu',
    'walker'         => new Custom_Walker_Nav_Menu(),
));
<