Sorry, I needed to be more specific on this…
This morning I found out that the WooCommerce Multilingual plugin is the problem:
When this plugin is enabled, I cannot access the created Custom Product Tabs anymore. It forces me to use the Woocommerce Multilingual utility (which is great as it’s own however!).
But when I temporarily switch-off the WooCommerce Multilingual plugin I can add the Custom Product Tabs for the other language.
I think I don’t need more help.
Cheers,
Jeroen