You can add a new "movie" [dummy movie!] with the title "Whatever Is The Name Of The Collection", and them convert that one to collection, then you can add all the movies that belong in that collection.

Collections are used to indicate sequels though, just for your information :)

You need to add a German translation for that collection (if that's the language you want to use to edit the collection). Just click this link and you should be fine: https://www.themoviedb.org/translate/collection/384142?language=de-DE