Its fantasy - how do we know that in that universe Okay is not an ancient elvish word meaning "forsooth brother verily you yet live"
Excuse you, it’s late middle Elvish of the Sylvan dialect, and it means “By my troth, though words proceeded past thine lips no intelligible utterance rang in mine ear but in distain for all appearance of discourtesy, verily, this is my stock response.” Also, on topic, anachronic language can put me off when it’s incredibly wrong, but in some ways our modern day vernacular and expressions are so ingrained with little thought, there are plenty of times I didn’t catch them entirely. Like my brother calling it an elevator instead of a lift or an apartment instead of a flat—he lived in London with his wife and so he had adopted all the jargon from that neck of the woods, but being back his vocab slipped black into being American again. It took a hot minute to realise the difference in his expressions, because I’m obviously more accustomed to hearing American terms even if I was accustomed to hearing him specifically using the British versions. Maybe in the same way I might say a Japanese word in a conversation with nonJapanese speakers because that word feels so natural to me. The big one is referring to my grandmother as “my obaasan” or “baachan.” Or I might describe the vibes a guy gives off as “sukebe.” All of this is so common with my immediate family that no one realises it’s weird in an otherwise completely English conversation. If I heard anyone else use those expressions outside our home, I don’t think I’d think twice about it—even if say the person is white or Pakistani or Peruvian or whathaveyou. Maybe much later, long after the conversation I’ll go “. . . wait a minute.” Sometimes I feel like reading novels can be much the same. So long as it isn’t specifically calling out technology or poignant slang, the naturalness of the words to me makes any oddities of it for place or time unremarkable.
Different experiences colour your reaction. I used to work in the pop culture industry, and I would hear non-Japanese say these expressions all the time. You know, ending sentences with "desu" or describing something as "kawaii". I'm not Japanese, but I speak it (not fluently, but well enough to have no trouble making myself understood in Japan). Hearing people use expressions they've picked up from anime without the slightest idea how to use them annoys the hell out of me. Imagine spending three days in a giant hall filled with people trying to talk like anime characters.
