Hi @jochen.walz,
Welcome to the Auth0 Community!
Yes, that’s correct! 
Using a post-login action is the correct way to append custom claims to an ID or access token.
Let me also clarify that the profile scope, while it should contain the zoneinfo and locale claims, in this documentation, it states that profile will only return the name, family_name , given_name , middle_name , nickname , picture , and updated_at claims.
See screenshot below:
This means that you must use a post-login action script to append the zoneinfo and locale as custom claims to your ID token.
Let me know if you have any questions.
Thanks,
Rueben