Welcome to Flyertalk, lukaszjur, and I hope that we will see plenty more of you here, thank you for your question.

For the EC261 component (which can't be 800€ for one passenger) then you can insist on payment in Euros, and then it will be up to you to pay your bank to get the final payment in PLN. For the fare component, then I would look to get back exactly what you paid at the time, in whichever currency you used. If there is a shortfall then you may have some further recourse, but I suspect that is down to what Polish consumer law and its remedies offers to you.

Something for further research is that every day IATA in Montréal posts a daily exchange rate chart, and for larger airlines this is the basis of inter-currency pricing - it's usually a fair rate of exchange that is used. So that's something you can investigate in terms of deciding an equitable outcome.