I downloaded the german installer, but the game is installed in english. The installer only let me choose betwenn english and france. Since I don`t speak france I`ve chosen english, but iI would rather like the german version.

It's an all language installer. Check for a config/language tool. Otherwise contact support.

Ok, will look for that next time I play!

The language selection when you install the game affects only the installer itself (see screenshot).
Once installed, start the game and in the "main menu" skip to the right, until the "option record" appears. Go to Game options and change the language to Deutsch ;-) See screenshot.

I tested it yesterday first hand, downloaded the current "German" version offered by GOG. After installation the game starts up in English. I skipped to the right until the record labeled "Options" appeared, opened it and went to the Game section. See screenshots.

You have to go to the options screen at the very beginning of the game. Do not press the "New Game" or the "Continue" button, but use the white arrow at the right side of the screen to switch to the "Chapters", "Multiplayer" and finally the "Options" screen. The in-game options screen which you can access from the pause mode does not allow to change the language.