Thanks for responding - here is a little report of my progress (I am using supl.google.com already):

Installing with English language selected seems to work better. I was able to reach the point where the lady sais "Let's get started..." and the route shown seems to be correct (I'm at home and not driving, so I can't really tell).

Trying to change language settings to Deutsch leads to the same sh... I experienced before. Because waze crashes immediately when starting to download german language files, I even tried to manually copy the files I saved on my computer to .waze/sound/deutsch, but that didn't help. There are 48 sound files in the german directory, I am not sure if that's all I need because they might be generated by an incomplete download.

I tried one more reinstall now keeping the language settings to english. Next time driving (tomorrow maybe) I will try to be guided in english language - I will keep you informed.