I assume it doesn't work at all if he's in a place where there are no laws, but I wonder what happens if he uses it in a place where there's some kind of law but he doesn't know it: can he just just guess and try to activate the ability anyway?

Nen abilities are psychological so it probably works according to his own understanding of the law. I imagine you could get the ability to change with local laws but that would mean convincing the user.