Character voices. It's a great method to work out details of their background and how they interact and their distinctive voices; set them to talking.
It is also a way to explore the idiosyncrasies of the Translation Convention.
The reader is going to read the book, dialog and all, in a language which is comfortable to them, regardless of what the characters might have been originally using. Thing is, this is one of those illusions that works best when it goes unremarked.
It may be an obscure example, but in the Japanese theatre the ninja-like garb of the kuroko stage hands is a signal to the audience that these people don't exist within the frame of the story. This approach reaches its height with the puppetry art of bunraku. On first glance the stage is crowded with people in black (three puppeteers are necessary for each puppet.) Blink, and most of them go away, leaving just the actors in the story.
The thing about invisibly accessible dialog is it is terribly easy to throw a stumbling block in front of the reader. The moment the reader notices a word choice, they are thrown out of that easy immersion. They stop hearing the character voices, and start seeing words on a page where an author has made choices.
The easiest way to throw the reader out is to use speech that sounds too colloquial. Never mind that "Okay" has citations going back to 1839, and "puke" was invented by Shakespeare; use either in a novel set much earlier than 1920 and your audience is going to stop, lift their eyes from the page, and ask, "Did they really say that then?"
What's funny is that this is not restricted to language that is plausibly correct for the period. (Strongly related to the Translation Convention is the unsaid understanding that dialog is edited for clarity and time; it is understood that no-one actually talked like that -- ever! -- but the reader is willing to pretend they do rather than wade through page after page of, "Um, well, the way we sorta did the thing, I mean the thing, um, the thing we were doing, was..." The same goes for overly exact replications of period speech patterns and slang.)
That is to say, an "Okay" can yank the reader out of a story set in Ancient Rome, too. Despite that the text is modern English, not Latin. As I said, the Translation Convention only works when the reader is able to pretend it isn't there.
Linguistic origins are another potential stumbling block. In my trunk novel Shirato the setting is an alternate world but the conceit is that the culture we spend most of our time with is speaking colloquial modern Japanese. So it is "understood" that the dialog on the page is translated from Japanese into nearest English equivalents. Then I had a nuclear reactor to describe and hit an interesting problem. They could speak of plutonium, and gamma radiation, but the blue glow of Cherenkov was off the table. Why? Because "Cherenkov" was obviously derived from the name of a Russian, and there's no Russia in the world of the story.
Of course there are no Roman Gods (Pluto), or Greek Alphabet (Gamma) in that world, either. The point of how the Translation Convention works is not that the words are somehow right or wrong, it is whether the words are obvious enough to the reader to attract their attention in the moment.
Randall Munroe pointed this out in a strip he did about Star Wars. Han Solo has just identified his ship, the Millennium Falcon, and Luke Skywalker asks, "What's that?" When Han starts in the Kessel Run story, Luke stops him, "No, I mean; what's a falcon?"
Of course many, many writers try to get around the terrestrial animal problem with what James Blish dismissively called the, "Call a rabbit a smerp" method. This is usually a failure. Even David Weber is guilty with his "near-bears" and "psuedo-corn" and whatever, despite there being good reasons for those terms in-world. It still stops the story while you think about it.
So is the related, "Hold your hippogryphs" attempts to localize. On a really good day, a character can yell, "By Toutatis!" or "Great Hera!" and it slots into the "interesting and different thing a character says that helps make them distinctive" place in the reader's brain. But try out, "A Flying Greebix in the hand is better than two in the Vorus-Fruit thicket," and the reader will find themselves contemplating your cleverness -- or lack of it.
It's the sort of thing that some writers and some texts get away with better than others (Sir Pterry was a master). And often there are no good solutions. If you write "Cat" then the reader may ask, "They have cats on Alpha Centauri?" If you write "Smerp" they'll wonder what that is. If you write "Neo-Cat" they'll also take note; which is a good way to intentionally attract the eye to the way things are different there, but in many cases to the local it fills the cat-shaped hole and even if it doesn't look exactly like a cat or meow exactly like a cat, it is, for all intents and purposes, just a cat.
In short, if it is there for color then, sure, give it an interesting name and remark on it. But if the point is getting from point A to point B so the plot can progress, it is better to call it a horse.
And then there's where the language is successfully transparent but is saying the wrong things. The horse of the late Bronze Age had yet to be bred and trained to riding; they pulled chariots. And the sheep had yet to be selected for sheering (bronze makes louse scissors) so they were more hairy than wooly. Not a real problem; if chariots or wool come up in the story you can explain then how it works in period.
Thing is, we have language to describe many of the places and things of the Bronze Age. But that language is foreign. Spear and shield and sword have become invisible enough to use. But what of, say, amphora? There are a great many understood Greek terms for things that were common in the late Bronze Age. But they are Classical Greek, in some cases originating in the archaic Greek of Homer, which is on the other side of the Greek Dark Ages from Mycenaean Greek. Which we don't really know; it is so poorly transcribed in Linear B most academic works chose to spell out the words rather than try to give a more colloquial transcription.
And many of the common words for people and things of the Egyptian New Kingdom are...Greek. Also Classical Greek. Again there is a transcription problem, as the older Hieroglyphs do not preserve vowels.
So there is a terrible balance necessary by the writer here on whether to try to use an anglo-saxon (or a franco-germanic-latinate loan word incorporated so long ago into English it merely reads as "English,") or a Greek word that has more color and specificity but might cause the reader confusion, or a best attempt at a Mycenaean term which would be nearly impenetrable.
This gets really basic. "Crete" sticks in the reader's eye because it is clearly modern. "Kriti" gets a semi-pass except that it is at best early Greek -- but I read a book recently in which the author chose that for his Minoan characters. (Oh, and don't get me started on "Minoan!")
What I meant by misleading above is that the more you use words from Classical Greece, the more you lose the distinctions of this earlier and quite different Bronze Age society. Just as if all you have is "pots" and "boats" it loses all distinctiveness as a culture.
(Yes, you could go on to describe the thing in detail, as I did with the horse and sheep above, but there are always tradeoffs. Sometimes you need the text to move more efficiently than that.)
And then comes those places where the easy, familiar, nearly-invisible term is saying the wrong things.
I was doing a trial sketch of a scene for the novel. There's a mild earthquake, and my POV character ascribes it to "The Earth-Shaker." Which is one of the sobriquets of Poseidon. Or perhaps aspects. And this already is a problem; that gods are rarely immutable points in most cultures. They have different local versions, different faces for different occasions, different personalities that are often treated as distinct entities. And they change with time.
It's a problem I had my characters point out in my Stargate fanfic; when you say, "Hathor is the goddess of fertility and music" you are making a mistake.
In any case, I chose "Earth-Shaker" because that fit the voice of the character. The Earth-Shaker can be identified with Poseidon, but this is not quite the Poseidon of Classic Greece. In Homer's time Poseidon was connected to earthquakes, the sea, and horses. (Sure, why not.) In the Mycenaean times the sea was more the bailiwick of other gods and Poseidon's role was closer to that of being chief honcho (although there's also a Zeus, and a female Zeus...and it gets complicated).
So, even though calling him "Earth-Shaker" is uninformative to the reader, calling him "Poseidon" can give the reader the wrong impression. Just like using any other Classical Greek terminology can give the wrong impression. And part of this is educating the reader; Athens always had an Acropolis, because an Acropolis is a hill (or, rather, the use of a height for a structure). That's the name, that's what it means, but the reader is going to be imagining the Parthenon up there and it has too good a chance of dragging them out of the story while they ponder when that was actually built.
As usual, there are no simple solutions. Worth noting that there are distinct approaches. If you start a book with a bunch of italicized words that are very much calling attention to themselves as foreign and/or technical terminology the reader will adjust and, in time, be reading smoothly again.
If you start ultra-colloquially, that also will vanish in time (although you still risk dragging the reader back out of the story when they notice just how much someone is sounding like, say, a pulp noir detective.)
The most invisible, and the way most writers go especially for English earlier than King James and all historical periods before that is to use slightly more formal language. Full sentences, an avoidance of contractions. This does become problematic when you want a character or a speech to stand out for being rough-talking; if your Roman Legate has instructed, "Have the men draw up in equal ranks, and we will proceed across the marsh to engage the enemy directly," it is difficult to follow this with the grizzled old Centurion turning to shout at his troops, "Roight, you lot!"