Greek
It's difficult to make distinctions like that with Greek, since there is so much continuity in the language. It is kinda like asking if the word "apple" is Anglo-Saxon, Middle English, or Modern English. Well, it is all three.
That said, the Greek phrases used in the game are in what you would call Classical.
As a native speaker playing the game, I can tell that the creators really did their homework. There are occasional incongruities like drachmae which is pronounced drachmee (as mentioned above, it should actually be a short e as in the word "get" and not long like in "greet"), and the pronunciation of some words even varies depending on which character is speaking (Alexios says "Irothotos" but others say "Herodotus"). Many of the voice actors were clearly Greeks (some I believe were Israeli based on the accent). Overall, the pronunciation was a delight. I was especially pleased that the game did not opt for the terrible "reconstructed" pronunciation of Ancient Greek used in most Western universities, which sounds terrible, and is not even accurate in light of modern epigraphical and linguistic evidence.
In terms of intelligibility, I would say any modern Greek speaker would understand around 90% of the dialogue. Some of the funniest sentences are used by civilians when under attack, e.g.
Πόρρω (Porro): Get back! (Literally: Away!)
Sometimes, there are anachronisms. e.g. the infamous malaka, while an ancient word, would not have been used like that. I also heard someone shout "τί λαμβάνει μέρος" (ti lamvani meros), literally "What is taking place?" "Taking place" is a modern expression copied from French "prendre place". An ancient Greek would have said "τί συμβάινει" (ti symveni?) or "τί γίγνεται" (ti yignete?), which also exist in Modern Greek.
The nicest touch of the game (my hat goes off to the Ubisoft team for this), are the sea shanties, the lyrics of which are taken from an actual ancient drinking song collection called the Anacreontea. The vocabulary here is a bit more tricky, but like any song, you can pretty much make it out after listening to it several times.
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