Originally you can only play the game with one language using the same language as subtitles. These instructions let you play the game with any audio/subtitles combination you want to use. Keep in mind that in some versions - German included - the cutscenes are not subtitled at all. I tried it only with English audio and German subs and there's no guarantee that the game won't crash later since I only tested it for the beginning. However IF the game should crash it's easy to revert the changes.

You need to get the subtitles and audio from the versions you want to use, download and install those versions in different directorys. In my case and the following example I downloaded the German and English version of the game. I installed both versions in different directorys. You will notice a subfolder 000 inside the game directory and in it the following files are important.

English version:
en.pak = English subtitles and UI
sounds.pak = English audio cutscenes
sounds_en.pak = English audio game

German version
de.pak = German subtitles and UI
sounds.pak = German audio cutscenes
sounds_de.pak = German audio game

You now simply just have to exchange the wanted files (i.e. for English subtitles and UI in the German version you would rename en.pak from the English version to de.pak and put it inside the folder 000 folder of the German version overwriting the existing one. To get English game audio you would rename the sounds_en.pak from the English version and put it in the 000 folder of the German version overwriting the existing one and to get English audio for cutscenes you would take the sounds.pak and put it in the 000 folder of the German version overwriting the existing one (no renaming necessary in this case since both version already have the same filename).

To be on the safe side: make a backup of the files you plan to overwrite so that you can revert those changes if necessary.