btw - You can use this website : projects.nytimes.com/census/2010/explorer to check many things about a particular neighborhood or area. You can look up everything from racial composition to the education level or income level.
Artur makes a good point about New England but unfortunately they are not as welcoming to strangers. She won't have that problem in the Mid West.
The people in New England are pretty friendly to strangers, especially since they get thousands of new college students every year. I lived near Boston and there actually was a small Polish community.
But nonetheless, no matter where you go in the U.S, you're bound to find some time of shop that imports Polish/Slavic food. There's always a Russian or Polish or Ukrainian store in almost every bigger city in America.
Either way, I think you can't go wrong with either the Midwest or New England. Look into major metropolitan areas and the suburbs that surround them.