Reported for: WooCommerce Multilingual 3.6.2
Resolved in: WooCommerce Multilingual 3.6.4
When using a default translation that’s different than English, in some situations, the permalink base gets replaced by the translation in the default language instead of English.
This causes product urls in the translated language to not work and return 404 errors.
This is fixed in WooCommerce Multilingual 3.6.4-beta1