Translation for invoice

How do I make sure the standard text strings are translated on the invoice?

I've copied the English language file for CBSubs, translated the most common strings, including the ones that I recognize from the invoice.

However, those standard strings stay English: Pos. | Qty. | Art. Num. | Item | Price, for instance.

Is it another file I need to translate?

Those strings are in the below language file.

/components/com_comprofiler/plugin/language/cbpaidsubscriptions-en-gb/language.php

You can find for example "Pos." on line 482. So if is translated it should show your translation.

Strange - I've already translated that and put it in its own language folder. That way all other strings have been handled for CBSubs, but not the invoice...

Any other ideas?

I've no idea, check all your language files carefully and ensure the string isn't duplicated. You can also enable language string debugging within CB > Configuration > Integration to help debug missing translations.