This page is not what the link you clicked said it was; sorry about that! When links in the fic you're reading don't go to a real-world webpage or to a fake social media post, they send you here instead, where you get out-of-universe author's notes.
Some additional thoughts on Armand and languages! The description in the fic of Daniel saying Armand is often speaking/reading Persian is a necessary invention to help explain why Daniel seems to think Armand is Shi'ite, but the Urdu he mentions Armand speaking around the penthouse is canon-supported: he's code-mixing Urdu and English in that one phone conversation we can kind of half hear. Urdu is the modern form of Armand's presumptive L1, and I'd personally argue (potentially against fanon? I've seen a lot of fics where he doesn't remember his L1/all the languages he speaks are notably European) that his current fluency in Urdu isn't a result of (re)learning the language to connect to his culture, but rather that he never forgot it in the first place. He says that French was his fourth "and poorest" language in 1556, after all. Maybe Amadeo was not the only Hindustani boy in the palazzo and he was able to stay in practice in secret, or maybe Marius was not quite as racist as he could have been. There's something charming about the idea of Amadeo attempting to teach Marius Hindavi!
Noor thinks Armand is speaking Armenian in 1x4 because, well. Devil's Minion cathedrals. For obvious reasons, it's not very easy to find Islamic materials in Armenian on the internet, and I don't speak Armenian myself, but I was very excitingly able to find an Armenian tafsir (Qur'anic exegesis) which calls 'Asr "Asr namazy" with that word order and declension. A win! This is surprisingly tricky to find across various languages, because while code-mixing in English it's common to say asr namaz, most languages actually do the opposite order, including Persian itself: namaz-e asr. The English subtitles (for me at least) wrote asr namozi, which is the way you'd write it in current-day Uzbek and seemingly further suggestion Samarqand may have come in between Delhi and Venice, though obviously Daniel wonders if it's Crimean Tat to wink wink nudge nudge us in the direction of the Armand reveal given his backstory in TVC.
I also feel like I have to say I don't think either Marius or Santino ever taught Armand Latin. Yes, the Children of Darkness performed Latin-language rituals, but that hardly requires fluency - how many choirs perform Latin songs they've just memorized by sound? I feel confident Armand doesn't actually speak Latin because when we hear him speak it in season one, it's... very bad. Like, not "poorly pronounced" so much as "that is not a real word", swapping sounds in a way that you would be unlikely to do if you actually spoke it, but very easy to do if you were memorizing a series of meaningless syllables. Which, like, likely Assad was, but we're not looking for a Doylist explanation here! It sounds to me like Armand has heard Marius or perhaps Santino say this particular phrase many times, and knows what it means, sure, but not because he actually understands it.
As far as my own headcanon is concerned, I like to think Marius primarily spoke Persian with Amadeo (Persian being the official language of both Delhi and Samarqand, and the primary language of religious poetry and song, they would both definitely be familiar in this world I've devised where Marius was involved in the Timurid Renaissance before the Venetian), even in Venice - maybe because it's already shared and that's simplest, maybe because it's easier to keep someone under control when you are one of only a few people they can communicate with, YMMV - and Amadeo learned either Venetian or the Lingua Franca with Riccardo's help. Latin, I think, would be restricted to stock phrases, including perhaps the vocative "ere" (nominative "erus"), which is my personal preference over words like "maestro" or "padrone" because unfortunately my Roman Empire is...the Roman Empire. "Dominus" means "master" in Latin, technically, but despite what shows like HBO's Rome would have you think, in actual use, "domine" (the vocative form) meant "sir", and was a polite form of address in general (and sometimes horny, as in "domine mi"/"my master" (voc)). Actual slaves addressing their owners would say "ere", which means "owner" or "master". This isn't really relevant to anything, I just... didn't have any way to bring this up in the fic proper and think it's fun.
ETA: Noor calls Arun a Westernization of Aruna because she's assuming Armand is fictional and Daniel went on a wikipedia list of mythological figures, picked out Aruna, and then dropped the -a so it would sound masculine in English. This is a pretty reasonable assumption for her to make, but for what it's worth, is not an accurate description of the linguistic developments which led to the actual modern Hindi-Urdu name Arun!