Well, obviously, knowing the language sufficiently to pray and sufficiently to converse in it deeply on various subjects are two different things.

To pray you actually don't have to know it that well. A Russian being able to say a prayer in Church Slavic doesn't know Church Slavic, likely. Same with an Armenian and a prayer in Grabar (though that's an archaic form of Armenian, so some sense will get through).

These are bad examples, because Russian is related to Church Slavic (though it's a South Slavic language) and Grabar is the classical form of Armenian.

But you can think, again, of a Pole or a Catholic German who can say a prayer in Latin, but doesn't speak Latin.

And knowing the alphabet is not the same as knowing the language.

So nothing obvious in that. Kids from Rabbinic families and in general with good Jewish education would, of course. Just like kids from good families in European societies would learn Latin, Ancient Greek, and probably also Hebrew.