I want to copy over custom product attributes to English and German language. These texts and values maybe the exact same Dutch text.
For some products 'specifications' tab is visible on the product page. For some not.
Example: no specs tab
Example: a specs tab but no values:
Sound like an issue with the custom product atrributes. Try creating some new attributes and see how that goes. Try using 'translate independently' option.
The recommandation is to use Product->Attributes (rather than Woocommerce's Add Custom Attribute), translate all the attribute values there (you can also translate the attribute name in string translation – if your taxonomy is called, say 'size', search for Attribute_size in the woocommerce context), and have the full product with variations defined in your default language before attempting to translate it. To translate, the best way is to use "Duplicate" followed by "Translate independently".
It appears it's a known issue and they are planning an update this month.