How can I edit submenu headings using wp_nav_menu walker?

Here’s one way to do it with the help of the menu CSS classes:

For the menu item that you want to modify the label of, add the wpse_menu class to it:

This is supported with:

 /**
  * Modify the title of menu items that contain the 'wpse_menu' class.
  * We restrict this to the primary menu.
  */
 add_filter( 'wp_nav_menu_objects', function( $items, $args )
 {
     if( isset( $args->theme_location ) && 'primary' === $args->theme_location )
     {
         foreach( $items as &$item )
         {
             if( in_array( 'wpse_menu', $item->classes ) )
                 $item->title = __( ' Related ' ) . $item->title . __( ' products ' );       
         }
     }
     return $items;
 }, 10, 2 );