He then told her he would be visiting Poland in the summer and would like to get a drink with her. When I saw the conversation we got into an argument and he told me that that's just how Polish people joke and that I didn't understand the culture.
This was probably the most stupid and non-sensical excuse that i had ever seen.
Your boyfriend is not just a bad-character. He is also dumb, extremelly dumb. Get rid of him.
Men (and Women) that cheat should be put to suffer and feel pain.