At this time the translate is possible using .mo/.po files only.
So I concluded that there would be changes within the .mo/.po file structure. -> I removed most source changes done via child theme. Those are the open issues:
Please take a look at  (private content):
– Within the url “portfolio” instead of “autorinnen” gets listed
– Within the breadcrumbs “projects” instead of “autorinnen” gets listed
One standard license is valid only for 1 project. Running multiple projects on a single license is a copyright violation.