international companies are not so stupid to give a Greek employee with a master about 500 euro per month in Poland.
Well, but why would they want a Greek employee with a master... ? Like people already said everything depends If they really need a guy fluent in Greek, with contacts in Greece, knowledge of local law, customs etc. If yes then you should get at least 1200 EUR, probably more but If they don't need a guy with these skills/knowledge then they won't give you even 500, they will give you nothing. Why would they even take a Greek guy not fluent in Polish ? To show customers as curiosity ?
Is the tax rate this high in Poland?
Income tax is not that high, ZUS and health insurence is the problem.
Is 6719 PLN a good salary in Krakow?
~280% of average salary. In Kraków a bit less but still at least 250% of average.