This app is one where it isn't just a matter of translating all the English messages to German. It works because they're averaging together (well, that's a considerable oversimplification) a whole bunch of different specific localized weather sources/networks that are available in the US, and their algorithms for the "averaging" are specifically tuned to those US resources. Sure, Europe has state-run and personal weather stations, too, but they're unlikely to be arranged/associated in precisely the same way as the ones Dark Sky is using in the US.
So, changing the on-screen text into German wouldn't be too hard, but rewriting their entire server-based weather collecting/comparing back-end to handle a non-US region would be a huge job - not just writing software, but doing the research into what all the weather station networks are and how to access the data, and getting the appropriate permissions to access all of them.