So, let's brainstorm some solutions:
1) The devs could hand out individual Steam keys for the free DLC to German players. There would need to be some way to reliably get into contact, maybe a dedicated support mail address.
2) As was suggested in a discussion on Steam, the devs could release a paid version of the free endings and make it available for sale on itch.io and/or keystores.
3) What about releasing s sort of "season pass" containing all ending DLCs, including the free one? The season pass could then be sold here on itch.io or in keystores, giving German players a way to acquire a Steam key for it.
It would be very much appreciated if a developer could chime in and let us know about their thoughts.