I learned how to say thank you for a great meal in polish before going to her house for dinner one time and she rolled her eyes at me and complained about my pronunciation.
I think this remark says it all about your mother in law... that is about character, and it will not change, unfortunately, so don't bank on it.
why should you make any efforts to learn polish, and please his mother, if she doesn't even know to value and compliment, to encourage you, but does exactly the opposite! Disgusting behavior.
I was in pretty similar situation: it is just that in our case it was even worse - neither of my parents in law accepted me... I also was not of the "right" nationality (in my case I was not german enough :-) ...), and I cooked in a wrong way, etc....... but my husband sustained all the attempts from their side to block our relationship, he just put it in a way - either you want it or not, that is my decision and you either accept it or ......
After more than 10 years of very complicated relationship with his family, I can only suggest few things.
If your husband will try to argue with his parents about you - it might not help much, but on the contrary, can lead to additional tension. Be ready for that. I also dreamt about my husband asking his parents to stop that bullsh..t against me, but at the end of the day when he did it - their reaction was like explosion...huge scandal, "how dare you? who are you? who is she? …" he ceased talking with his parents for a few years after that incident.
It is not to say that the same “show” must be in your case, but just to warn you that sometimes people react very heavily...
If you feel unbearably bad when meeting with his mother - just reduce your communication with her to the very necessary minimum. If you are really interested to keep your relationship with the current boyfriend, just accept that those are his parents, as they are, and you stop worrying about what they
think about you. The most important is only what happens between you and your boyfriend.