Noam Chomsky once wrote that it is part of the
“human essence to be capable of learning only the languages whose syntactic rules satisfy the constraints of certain linguistic universals”. 
Of course we need to accept such a stipulational definition of the sortal [human] in order to make sense of humans at other possible worlds. And even if we accept Tony Blair via the sortal [human] at other possible worlds, the possible entities would still not be Blair qua Blair at these worlds. It's essential that Blair - via the sortal [human] - is capable of learning language L, or even Blair qua individualised particular (to use Leibniz's term) that he's capable of learning language L. If Blair had existed at another possible world only via the sortal [human], it wouldn't actually be Blair at that other possible world because what makes Blair, Blair, is the individualising of him as a particular.