You can look at the price list here: https://www.dwd.de/SharedDocs/downloads/DE/allgemein/preisli...
All the important stuff is free.
So one would just have to develop a free app to process and display this data ;)
Basically, the data is free, but most higher-level processing results are not (like point forecasts derived from the models).
It’s high quality data, for free, you’d just have to build your own frontend.
You can look at the price list here: https://www.dwd.de/SharedDocs/downloads/DE/allgemein/preisli...
All the important stuff is free.
So one would just have to develop a free app to process and display this data ;)