Sounds just like any other API, from most REST APIs to messengers.

HBCI is a lot better designed, and a lot easier to work with than the many different messengers that exist on phones nowadays. Everything is documented, everything is specified, and you have a stable API. Try getting something like this for Facebook Messenger, Allo, Duo, WhatsApp, Signal, Riot, all at once.

HBCI is bad, but it's a rare gem. Usually, when many major corporations have similar products, they try to prevent any such APIs from being created at all.

> non-machine-readable

I'm not sure I've seen any major API that's machine readable. Most major APIs don't have any machine-readable specifications publicly available. They probably have them internally, but almost never publicly.

> german-language-only

That's not really an issue, if you're in Germany. I don't go into the US and expect their banks to provide their specifications in English either. I realize that English has become the lingua franca in IT, but that's not a good thing.