The way that our translation works, everyone will still technically be speaking the language they speak normally -- their first language. They will hear everyone else speaking their presumably different first languages as well. The way that translation works is that, though everyone hears different languages, they can understand all of them -- universal translation is on in the mind rather than characters being forced to speak a random universal language they don't actually know.

So, if a character was already familiar with Russian, they would know Jay was speaking it, but they would understand what he was saying in it. They wouldn't just automatically know what the language was or where it came from, though (someone hearing a troll speak wouldn't immediately think, 'oh! Alternian!' if they didn't already know it was called that).