Some written languages have been forgotten by humanity and were later “deciphered” by linguists. Examples are Ancient Egyptian, Classical Mayan, and Sumerian. Other ancient languages were never totally forgotten, such as Ancient Greek, Latin, and Sanskrit, and are well known today.
Do we know enough about the deciphered languages to allow us to, in theory, compose arbitrarily in them? For example, there have been attempts to translate modern popular literature into Latin. Could one, with today’s store of knowledge, translate Harry Potter into Sumerian and stand a decent chance of making a faithful translation (of course, some leeway is given to coin words for concepts the Sumerians didn’t have)?
Alternately, could we, in theory, set up an organization where the administrative language used is Sumerian, and expect the people that we have taught Sumerian to to be able to speak, listen, read, and write in that classical language and get real work done?
The first problem is vocabulary. For languages that only exist as written texts, we only have as many words (and grammatical formations) as are in those written texts. Not only can they not come close to the full vocabulary of the language at that time, but there will be all kinds of words–for technical gadgets, for example–that never did exist in those languages. These words would have to be made up, for the language to be usable. This was done when classical Hebrew was turned into modern Hebrew.
This is frequently done with Latin – modern translations of modern texts into Latin are filled with Latin neologisms for car, airplane, etc.
Have a look at Cattus Petasatus or similar works:
Of course, if you want to introduce a neologism into contemporary Latin, get ready for a fight, albeit an erudite one. But in general, the ones that get used have passed some tests of “authenticity” that modern languages don’t subject new words to because a living language creates itself. A dead language that lacks the dynamism of a living language or the force of data of a well-documented ancient language could not take be innovated with any meaningful authenticity. Then again, it’s not like the bastards can stop you.
I have a copy of Peter Rabbit that’s been translated into ancient Egyptian hieroglyphics, so someone out there’s got some time on his hands, I suppose.
Hands down, though, you can’t beat Elvis in the original Sumerian