Replace the “widgets” label in the backend

Finally, I used this solution to change the “Widgets” label in the sub-menu (Appearence > …):

add_action( 'admin_menu', 'adjust_the_wp_menu', 999 );
function adjust_the_wp_menu() {
  $page = remove_submenu_page( 'themes.php', 'widgets.php' );
}

add_action('admin_menu', 'register_custom_menu_page');
function register_custom_menu_page() {
  add_submenu_page('themes.php', 'custom menu title', 'custom menu', 'add_users', 'widgets.php');
}`