Okay is NOT okay

I’ll put my hand up. I go to writer’s forums. I particularly like Mystical Adventures . There are some others, too.
I started going there a few years ago. I wanted to discover the secret of getting published. Now I know there’s no secret, just hard work. But I hang around anyway.
But there is one discussion on these forums that always bugs me. Here’s how it goes: someone shows a piece of writing, set in a FANTASY WORLD. ie not Earth. Someone else comments on it, some good, some bad, and then they say “in a world like this people would not say okay.”
I mean, WTF?
I mean, really, WTF?
This is a fantasy world. No one here is speaking English. Everything is translated. So why would you translate it into English that hasn’t been used in 300 years?
You wouldn’t, right?
Ah, but you want the language to be authentic. Of course you do. Try this for authenticity:
The earliest use of the word Okay in print is 1839, March 23rd in the Boston Morning Post. In the first eight instances of the use of the word in print, 5 didn’t have explanations beside them. This points to the word being in use for speech for a while before this.
The etymology could be from Oll Korrect, which Americans used to misspell because they thought it was funny, or from the Chocteh word Okeh, or from the Wolof waw-kay.
“Oh ki, massa, doctor no need be fright, we no want to hurt him.” (David Dalby (Reader in West African Languages, SOAS, U of London). (1971) “The Etymology of O.K.”, The Times, 14 January 1971)
Okay, now the word boredom.
The phrase “to be a bore” had been in use since the 1700s, but the word boredom was first used in the novel Bleak House by Charles Darwin. In 1852.
Yes, that is 13 years after the first use of the word OK. So, can we now not use the word boredom?
Of course not. We can use any words we want.
The important things in a story go: character. plot. That is it.