We are just in the process of getting a complete German language translation of the game done, I’m sorry I’ve only just seen this thread… 
Any other language planned?
It depends if people buy the German version, getting a pro-translation is pretty expensive. Plus I need to code in support for some special characters…
I’m trying to make Italian translation: at the moment overwriting the core files is the only way? → viewtopic.php?f=37&t=8731
Is it possible, to switch to German free of charge if I already purchased the English version, or do I have to buy the game again in German?
I bought the English version from GOG.COM
If it will be free to switch to German version, then you are free to use this translation as a proof reading guideline.