Should be fairly straight forward. You can also change the dip switches if you like, but these can be overridden by game settings.
http://pinwiki.com/wiki/index.php?title=Williams_WPC#Country_DIP_Switch_.28or_jumper.29_Settings
EDIT...If you want full German speech, you have to change the sounds ROMS also. https://www.ipdb.org/machine.cgi?id=3685
DISCLOSURE: I have never done this but I may try it