I was thinking about this because...
of this little section in the book "The Ancient Languages of Mesopotamia, Egypt, and Aksum": The Status of Sumerian in Antiquity
in the chapter about Egyptian, it says "Late Middle Egyptian coexisted with later Egyptian for more than a millennium in a situation of diglossia (Vernus 1996:560-564)." [The citation leads to a French, or at least non-English, source.]
that oft-repeated fact about how ancient the pyramids were already to the Romans
I made the same post in /r/linguistics first, and it was suggested that I also post it here.
Emperor Claudius was able to read Etruscan, wrote a treatise on the language and an Etruscan dictionary. So yes, some Romans at least did learn ancient languages.
From Claudius and the Etruscans by Keith Armstrong.
The Achaemenid Persians spoke and (more often) wrote in a bunch of languages that were older than their civilization. Old Persian (their own language) is only attested in inscriptions from as old as 525-550 BCE, but the Zoroastrian texts were written in Avestan (which is from the late Bronze Age) and although it wasn't spoken very often, Akkadian was very common in writing (most inscriptions are bilingual in Old Persian and Akkadian).
A more general answer to your question might be Aramaic, which was lingua franca in the Near East beginning with the Neo-Assyrians until Alexander. It was still pretty common after that and there's even been Aramaic inscriptions found on Hadrian's wall.
Sumerian continued to be used in administrative, legal, and royal writing, usually alongside Akkadian and its descendants Assyrian and Babylonian, and also as a religious language, for over a thousand years after it ceased being used in everyday life by most Mesopotamians. There is evidence of it being studied by at least some people as late as the Hellenistic period, where there are tablets with Sumerian words written with Greek letters.
We have some interesting bits of text in a very old Latin, among these the Saliar Hymns, which are traditionally attributed to King Numa, legendary second king of Rome, i.e. the guy after Romulus, 753–673 BC. By the first century BC, Cicero, arguably the most educated Roman of his time, couldn't read them.
Heretodus tells some great stories in his book "the histories" - A collection of stories he picked up while traveling around the ancient world.
He tells the story of some Greeks trying to figure out which languages older, Greek or Egyptian. If I remember correctly the story goes they took a baby (or babies?) and raised them in complete silence until one of them spoke a word. This word was meant to be either Egyptian or Greek and would prove which one was the oldest.
You should really pick up a copy of his book, it's very entertaining reading and an awesome snapshot of life in the ancient world ( he hilariously over exaggerates a lot of stuff so get an annotated version that clears up what he may have gotten wrong)
Just an observation. Reading old English is hard, Chaucer or going back further to Beowulf.
Did Latin or Ancient Greek drift so much? Were people studying archaic Greek and Latin then (back in the heyday of the languages) in the way that people still study old English today?
