This plugin is now discontinued and has bugs, so I would strongly suggest that you use WooCommerce Checkout Field Editor Pro instead.
How to translate WooCommerce Poor Guys Swiss Knife labels with WPML
- Go to WMPL > String Translation.
- Check “Auto-register strings only when logged in as an administrator” and press Apply.
- On your site’s frontend, visit the checkout page. Wait until WPML scans for page strings.
- When the page has fully loaded, go to WMPL > String Translation and only the strings within the wcpgsk domain.
- You are now ready to translate them!
How to get a custom field using PHP functions
$my_custom_field = get_user_meta( get_current_user_id(), 'billing_custom_field', true );