For Retail: Extract german.lang to c:\program files\amnesia the dark descent\redist\config\lang_main
For Steam:  Extract german.lang to c:\program files\steam\steamapps\common\amnesia the dark descent\config\lang_main

Make sure to save (or rename) the original german.lang in a nother folder, just to be on the safe side!

If you have any problems, feel free to contact me at marcel.thiz@gmx.de