I had a friend just get back from Germany and they now have a Windows 10 OS that uses a combination of English and German. They said when they were over there, a German tech used a German Windows disc but used their US/English key to reinstall their OS.
Now as you can see with the image below, some text is in German and some text is in English. When Windows 10 is starting up and says Welcome for example, theirs is in German and says Willkommen.
I checked the Region and Language Settings and didn't see anything about German being set as the language.