thanks for your help.
The parameter to select the language worked. But if it is used together with the locale-parameter, the German umlauts are broken. Not so important for me, since I can now execute scripts like ./install.sh without finding the right keys by trial an error (umlauts are rarely used).
Your other hint regarding the partnew-command was good. Although I read about it the evening before my post, I didn't used it because it just looked like a nasty workaround. But I tried it and it worked. It is still a nasty workaround since partnew creates a new partition