Hey, Thanks for your time and your research. On the links you send me the salary is around 35K a year as a median (which includes 10+ years of experience developers as well as juniors) this is what we offer too (+super cool start-up, trips to the US, equity …).
I understand though that you have doubts 
I would probably feel the same way. One more thing though, software engineers in France are people with a Master’s Degree in Computer Science and these people rarely do web development. This is completely weird, but they think web development under their skill level and go to hard mathematic oriented software dev instead of web.
Well, that being said I am glad I learned more about Poland and I am glad to know you get the salary you deserve!
Best regards, and good luck for the future.