White screen is almost always busted php. I've installed the German pack about half dozen times, and never had any problems at all with the German pages or with the German shopping cart or with the German checkout. Then again, I followed the install instructions precisely and did not do any type of switching with English and German directories as you have described. I would not know where to look first to try to help you solve your problem, other than saying a clean install of the German pack following the install instructions might be a good starting place .. and not using any modified php files that are not part of the Pack or not part of the instructions.

Frank? What say you?