I played the hell out of Gothic 1 & 2 back in the day, in it's original German version. I noticed that it's only available in English on GOG, but since I'd like to replay Gothic II I wonder if it's enjoyable?

Yes.

I've never played the German version but the English always seemed very high quality to me.

The German version is the far superior one, but the same can be said for many games that are translated into German here. Games like Two Worlds (originally in Polish or English?) and The Godfather really do suck in German.

I agree, the german versions are better than the english ones, expecially in GII. There are still english test versions available, where you can take a look before you buy it.

I think that is true of most translated games - the producers put all their work into the original and know the language well so there are built in nuances that get lost in later translations. Plus, some of the translations in this one are just plain bad - phrases I've never heard and sometimes phrases that I had absolutely no clue as to what they were trying to say. It was rare, though.

While I've no doubt that the German version was superior, I was impressed by some of the language subtleties. Maybe that's generous. But the translation was good enough that comedic moments weren't lost. Example:

Dragon Hunter Thug: "Hey, I don't like people sticking their noses in where they don't belong. Can ya get that through your skull?"
Me: "Nope!"
Dragon Hunter Thug: "Then I'll have to beat it into you!"

I reloaded this scene multiple times because the delivery was just perfect. The nameless hero's "Nope," specifically. I don't know if there is a German equivalent for "nope."

The only problem I had was in the Temple of Adanos in the expansion. There were some puzzles that I think were supposed to be word puzzles, but made no sense at all. I wonder if they were poorly designed, or just poorly translated.

IMO both applies here. It took me also some time to get the clue in the original german version ;)