It has been discussed a lot in railway forums, it seems to be a fact that it's rather hard to buy the ticket if you do not live in Germany.
* the default payment method used by 99% of the sellers requires a German bank account
* most sellers request a German credit score (Schufa)
* many apps are not available in the app stores outside of Germany and of course none is available in a FOSS store
Haven't followed it recently, but I have still to see a success story that someone not residing in Germany has actually bought it successfully.