Ok, things got clear a bit... It turns out that if you buy a device from a manufacturer (e.g. VeriFone) it comes "clean", so you have to program it all by your own and satisfy PCI Compliance. Therefore, you have to be certified developer... at least for VeriFone devices.
However if you order a terminal from some kind of 3rd party provider, for example SIX, or Polskie ePłatności (one of the providers in Poland), it comes with some kind of, let call it "firmware". This "firmware" will, or at least should provide you an application for performing cashless transactions. It also should come with handy communication protocol. Of course you'll have to sign some kind of NDA.
In short:
If you want to integrate with a credit card terminal just call a local credit card payment service and ask them to send you a terminal you want to integrate with. Keep in mind that every country has its own transactions centers so your solution will be country-wide.
