As jnavila already noticed in this post, the structure of the German repo is broken.
I cloned the progit/progit2/master branch as a template to my fork and transferred this structure into my "de" branch. Then I inserted the content of "progit-de/progit2/master", as far as it was still up to date, into this branch.
Is it possible, before I start a PR, to check if any preliminary work needs to be done?
The present status is:
files in the root translated + 2 additional files
files in /book (not the directories) translated
chapter01 translated
chapter02 partially translated
chapter03 there is an open PR from "Sternenstauner"