Vampires would probably agree to a single language, at least in large geographic swathes in Ye Olden Times, for the same reason that there are agreed upon business languages and science terminology in our world: it makes it easier to communicate exactly what you mean to communicate and come to an agreement.

Yeah, that's also true!

Thanks for this, I'm really still building everything and can use all the input and info I can get on little-but-big things like this