I don't normally post in this forum, lol. However, I ran into this problem at work recently, so thought I could share my experience.
The most important thing that Cast Members are trained to look for is that the ID is in a language they can understand (generally English, but depending on the Cast Member, other languages), that it has a photo of the holder, and that it readily identifies the holder's date of birth, as well as the expiration date of the document (generally, expired documents cannot be accepted).
For German citizens, the Personalausweis is sufficient, as it is printed in English (in addition to German and French), and contains readily identifiable Date of Birth and Date of Expiry fields.
On the other hand, this sometimes creates problems for persons presenting European Union-format Driver's Licenses. The licenses do not readily identify the Date of Birth and Expiration Date on the front, as per the Directive of the European Parliament and of the Council on Driving Licences.
Licenses issued by the United Kingdom and the Ireland, both Anglophone countries, generally include an explanation of which number field is which on the back in English. However, most other countries publish that explanation in their national language, which can pose a problem at times.
Most Cast Members do eventually learn which field is which from repeated experience, but it can still pose an issue from time to time.