So on Steam this game is on sale currently 50% off!

Now, I don't care too much for Steam and I usually buy any game I can on GOG over Steam, even if it's a bit more expensive here, both to support GOG and to get DRM-free experience.

However, 50% off is quite a lot, and I am really tempted to "betray" GOG by picking it up there. If GOG had a 50% off sale here (or even 40%) this would be a no-brainer.

Come on GOG, can't you match what Steam is doing? It is your own game, after all! :)

Game and expansion pass (GOG keys) are currently 50% off on amazon.de. You could buy them there.

That's pretty neat. Is it the physical retail version? And will it be in german or is it just a european version?

Still though, I'd love to have it here on GOG and it would make sense to match the current Steam sale.

Witcher 3 was 50% off the last 2 sales on GOG too (fall and winter sale). You are just a bit late.
Now your questions:
The base game can be bought as GOG-key or retail version (GOG key too) on Amazon.de. The Expansions pass is only buyable as GOG-key. If you redeem the key you can download and play the game in all available languages (like with all games on GOG - same on Steam).