Yes, registering a .de domain as an Austrian citizen or Austrian company is possible with internex at no additional cost.
If you don't have a German address for the Admin-C, we offer our customers a free trustee-service.
The trustee-service in this case is a legal entity that represents the interests (the fiduciary duties) of the client. This entity also provides an alternative address in Germany. Of course, you, the client, remain the owner of the domain in the background.