I have found many other strings not translated to german: In the admin menu under Blog and Content the items are in english. If i click to “All Articles” i see many english strings.
I see. I think this is Tailor. One way could be to search for the string in your project through vscode, find out what is refering the string, then look the path of the file, it should give you an hint in what repo the modification should be done
Why are they custom files? These menu and submenu items are a part of the product. As i wrote before, this is a fresh installation without adding anything at all.
Technically they are not part of the product, they are demo files for you to customize (or delete).
The files are seeded custom files that come with the demo theme. You can delete them in the app/blueprints directory. The source files can be found in the themes/demo/seeds/blueprints directory.