> Isn't that common in Europe, even required by law in most of it's countries? I'm not sure about Poland though

Yes. I live in the Netherlands and we have this. I am a software engineer and working in a free sector (meaning I don't have a union or common branch contract) but it's normal that overtime pays extra. It's also the reason I rarely work overtime. Plus it's fairly common to be compensated in time. So if I work on a Saturday I get 1,5 off on my paid vacation sheet. Sunday pays out 2.