I’ve met second generation Polish-Americans from Chicago who came to Warsaw for intensive Polish courses. What was funny was that I could pronounce polish better than they could :-P
Possibly the Polish-Americans’ ancestors were speaking regional dialects that differed from the standard language. When Jimmy Carter visited Poland, he had a translator whose Polish was not the standard language, which resulted in some embarrassing mistranslations (Jimmy had “abandoned” America and “lusted” after the Polish people).