Bah, those were exactly the files I was looking for (already been answered on russian forums) but I never thought of looking into data.kfs, idiot.
Thanks for pointing me in the right direction, seems like items are stored in tex9.dds so it should be easy enough to add new icons now.
Oh an what I was trying to say, make a new language file for your edits (something like de_!!fixes.lng), archive it in a custom KFS and put it inside the mods folder. That way your changes should take priority over other language files currently in the game (and you can share it with others if you want).