Is there a way to change the voice acting to German but leave everything else in English?

Sadly not.

I guess I won't be buying it then. I don't like the English voice acting for this game. :(

Since Deponia is using the same game engine as the Edna & Harvey games (I think?!), maybe this could work:
Edna&Harvey GOG Forum post

Unfortunately I removed my copy of Deponia 1 before I could try that. Perhaps anyone else wants to give that a shot.

Doesn't seem to work, unfortunately. :( The folder structure is different compared to Edna & Harvey and I couldn't find any file(s) containing subtitles. The movie files in the English version even had German audio, so I suppose the game plays the English audio from somewhere else, which makes things even more complicated.

I am, however, very tempted to buy E&H from GOG even though I already have the English retail version. Didn't much care for the English voice-overs.

That's a pity! But thanks for trying that out! But I'm sure the programmers already registered that there a quite a lot people who want to play with mixed audio / subtitles. Let's just wait and see what they're gonna do about it.

They've just released german patch, so yeah, you can get it in german. I personally really liked the english version, voice acting was superb and funny

But that's just to get the game in German, right? (Deponia 2 didn't have that before.) Still no way to get original voices and English subtitles, though.

And I agree that the English voice acting does seem okay in this game. I look forward to playing it, I just prefer to experience the original dub if I can. :)