[MMM]

No. There is no tension between Poland and the US, so it's a dumb joke based on an outdated stereotype. Most Americans have never met a real Polish person, and if they did, they wouldn't say something like that.

Up until the most recent generation there has been high tensions between Koreans and Japanese, but at the same time, many Koreans live in Japan (often 2nd or 3rd generation).

To make a long story short, it's a racist joke that is reserved for your friend's "closest friends" but not one that I have ever heard or would expect to hear out of the mouth of any of my Japanese friends. If someone says something stupid, you call them "baka" not "korean".