When I install the German language pack on a fresh installed "Divine Divinity" (without the update 1.0.2.13, if I want to install this it says: "Update already applied"), I get the following error: "Installed version is not compatible with the patch". This doesn't occur with the French and the Russian language pack. I've already redownloaded the language pack and the setup file, but to no avail. Although actually the game seems to be converted to German language, can I be sure that everything is alright as long as that error occurs?

Yes, the patch installed correctly if the game is in German.

Did you start the game or run the configuration program before installing the language pack? The original language packs were tested against a clean install without starting the game, and would give an error (at the end of the install procedure) if they found unexpected configuration files (which the game creates the first time it is run). GOG was going to update the installers to fix this, but maybe they missed the German language pack, or have not gotten to it yet.

The full installer is the latest version of the game, and doesn't require the patch (meant for the earlier 1.004/1.005 GOG releases).

No, I didn't run the configuration program or start the game before installing the language pack. Seems to be that GOG didn't fix that yet.

Did you try the game afterwards? I installed the German patch over the English version with the music fix, and also got a message at the end of the install that it was incompatible, but the game itself is now in German.

You're right! It's in German, should have checked instead of believing the installer...haha

I did the same thing checking something when the language packs were first released (ended up deleting the game and re-installing without trying it, then had to re-install the language pack, then delete and re-install the game again).