German has to be used if a single German employee wants that.
I'm not sure about this statement. See [1]:
"The Regional Labor Court (Landesarbeitsgericht - LAG) of Nuremberg had to rule on a motion by a works council, in which the latter demanded that the discussions held with it be conducted only in German. At the same time, it demanded that all communication between the branch management and the employees be conducted in German. The Regional Court rejected both motions of the works council."
[1] https://www.heuking.de/en/news-events/newsletter-articles/de...