go to swiftkey > language > add language and it looks like simply adding a german keyboard should do the trick.
It says:
Multilingual: You can type in any installed language that is supported by your current keyboard layout without the need to switch keyboards.









I remember that I tried it a while ago and there was something that prevented me from actually changing. However, I tried it again now and although I don’t remember what that feature was it doesn’t seem to be missing anymore so I think that I’m going to try using it from now onwards!