Who is making these claims on Poland?
An NGO which claims to represent all the Jews: those who lives, those who died, and those who may or may not be born in the future or even those who consider converting to Judaism in the future :-P I'm making fun of it because it sounds so absurd that one can't take it seriously. The threat is real though.
Basically, this whole claim is based on some form of religious tribalism - which is obviously not represented in an international law - which ignores nationality and citizenship of Polish citizens who's got killed by Germans during WWII. That's exactly what Norman Finkelstein called a "holocaust industry"; a very shady organisation. Their claims are not only legally baseless but they sound more like an attempted extortion, which is a criminal activity.
The families of legal owners, of any nationality, can claim their lost property in a regular Polish courts. There are cases in which they won but the claim needs to be legal and well documented.