Becoming a CFI in the USA as a non-US citizen
You do not need to be a citizen to be a U.S. CFI.

You need to have US ratings, so you would have to have all your German licenses/ratings converted, which takes some time.

You have to be absolutely FLUENT in both spoken and written English.

If you want to charge students money for your services, you have to have US residency or other work visa authorization. You do not need a work visa if you train students for free.