Never, ever make an app around a closed-source API without written permission amounting to a license from the provider unless:
You enjoy having to explain to users why the app they're depending on broke and there's no documentation to tell you why, leaving them in a lurch for long periods of time (see: WhatsApp for webOS)
You have really good legal representation if/when the data provider decides to send a few threatening letters your way from its legal team
Thanks for the advice. It's more for improving my own development skills and it would never be a commercial project. We don't have many webOS users in our small country. I would be surprised if there would be 20+ people using the app to be honest.
API doesn't change much, it's just the most convenient app when you get around by public transport a lot here in NL.