What are the standards/expectations from a teaching postdoc contract? Recently, I talked with a CS Professor regarding working in his lab as a postdoc. He said that he is interested in my CV/profile, but he can only give me a contract as a teaching staff until he gets the grants next year. However, he said that all the courses he currently instructs are given only in German (which is a constraint for me).

But I was wondering if I'm supposed to do teaching as a postdoc, I shouldn't be expected to act as a TA of his courses. Instead, I should be the instructor of a course and not his own course. At least, that's what I always imagine as the level of teaching responsibilities for a postdoc researcher. 

Then, if that assumption is correct, I wonder if I can suggest a few topics that fit his group's research profile and is not given by other groups so that I can instruct in English. For instance as a course (or block course or seminar) in the international track. But, I like to know how realistic/practical this could be before discussing it with him.
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.