I don't think it even matters whether it is public speech or not.

The constitution guarantees free speech only insofar it is not limited by "provisions of general laws, in provisions for the protection of young persons, and in the right to personal honour".

And http://www.gesetze-im-internet.de/englisch_stgb/englisch_stg... sets forth some pretty serious limitations.

Nowhere is it mentioned whether any of this happens in private or in public.