What are the standards/expectations from a teaching postdoc contract?
If you are going to a new country (assumed based on the German tag and the language being a challenge) or even only a new university, then first acting as a teaching assistant can be very good - you will learn how courses are organized, teaching handled, what you can expect of the students, possibly some of the bureaucracy and the online systems that are in use.

The alternative is to learn all of this while, at the same time, creating a course or mastering the contents of an existing one, and learning all the involved bureaucracy. This can be done, for sure, but it will be involved and consume much time.

This probably also depends on your teaching experience: If you have already lectured a course, or have extensive experience as a teaching assistant, then you should be more prepared to lecture a course and can ask for it.

Consider also your language skills. Giving a comprehensible lecture in a foreign language is significantly easier than a free conversation. There will be an accent, but if you write and speak everything, many students can cope. You will need to find a good resource for learning the vocabulary and how to pronounce mathematical expressions, though, if do not yet have such.

There might also be intermediate roles between a teaching assistant and the main lecturer - I had such a position in Norway (at NTNU) as an øvingslærer.