But in the medieval period, when Latin was actually used as a lingua franca in just the way you suggest, it continued evolving, not just in vocabulary, but also in grammar and pronunciation, to the point where Latin users from different countries struggled to understand each other. If Latin today is really unchanging it's probably because there are few people who aren't classics scholars or in the Vatican who actually produce anything in it.
> If Latin today is really unchanging it's probably because there are few people who aren't classics scholars or in the Vatican who actually produce anything in it.
That's probably a big factor. Maybe Esperanto would achieve similar stability if it were only used by ISO working groups or in other official environments. But if that happens, it ought to come under fire for its Eurocentrism, its sexism, and its general quirkiness, all of which it has been able to skate on because it's never been taken seriously before.
