Well French isn't a bad guess, is it, considering she moved to France, married a Frenchman, became a French citizen, took his French name and probably spent most of her life in France?
Ask people what nationality Einstein was and many will say American, despite his German name, which is also not necessarily incorrect.
That doesn't change the fact that most people who learn about Curie or Einstein at school probably learn about their origin. That people don't remember everything they learn at school isn't particularly remarkable.
As for polonium, it's practically a pub quiz question who invented it and why it's so named.
> the Polish part of her surname is omitted from nearly every mention of her I saw over here so far.
Where her Polish name is being "omitted" I suspect it's more for reasons of simplicity than any 'Western' conspiracy to keep Poland down.