This is the non-commercial Hyyan WooCommerce Polylang Integration rather than the new Polylang commercial extension for WooCommerce, but the principles should be the same.
Coupons were not created as separate posts but Products are, and Terms etc are translated.
I’ve just checked in the coupons extension into this branch – there’s a coupons.php file:
Note also documentation:
I just took a quick look at WJECF_WPML.php now you mentioned it, and I think it’s already covered within the woo-poly-integration 1.0.2 but your feedback/suggestions welcome.