Actually, Patrian is like Latin in the middle ages. You probably guessed that much, but that means that most of the educated/aristocratic classes speak or read it in most of the non-new nations, but few of the commoners do, in any nation. So it would function, I assume, as a common language for elites, even if it isn't actually in use anywhere.
Though the time from the Age of Magic until contact is re-established in the Age of Rebirth isn't that long linguistically, so most descendant languages are probably not that different, and commoners from each land share many words and syntax in their dialects, making basic communication possible, though for complex ideas, or any novel concepts, translation is required.
), change in the environment in which the speakers live (a formerly aristocratic society could have a very wordy, formal, poetic language; but when the Age of Ice hit and they were forced to scavenge and hunt to survive, that language would probably be reduced to something more basic, for simple and quick communication - as with tribal societies in the Amazon whose languages have evolved to include or consist entirely of the sounds of the animals that live around them), and many other possible scenarious. One thousand years changes a language quite dramatically regardless of contact with outsiders; and with social changes like that, it could change pretty drastically.
