I want to put a type of swords in my fantasy world that looks like or is Japanese sword "katana". My question is: should I write that IT IS 'katana' or just describe the way it looks(the way "katana" looks)? For example if I just wrote that that group of people uses "katanas", wouldn't that be off-putting to read because the world is fictional and "katana" is a real thing? Just "sword" on the other hand is a broad term. To add to it "katanas" would have some sort of magical glowing properties.
Use another name
The work "Katana" describes a single, very specific sword design.
(odds are that what you believe a Katana to be, is actually a nodachi)
But more than that, it carries a strong cultural association. Anyone who hears the word "katana" automatically also thinks of Japanese Samurai, even if there are absolutely no further facts to support this image.
For a contemporary, western equivalent: It's like saying your characters ride around on Harley Davidsons, when you actually mean they drive around on large street cruiser motorbikes. Everyone will immediately recognize the name, associate and image with it, and then you are stuck with that image.
Better to invent a new word, or re-use and older word that is accurate but no longer in common usage. Or just describe the sword. "You see those redguards from Hammerfell? They’ve got curved swords. Curved. Swords"
This leave a much more lasting impression that just saying they use Scimitars, but still leaves some elbow room for alternate implementations by the author.
Fantasy authors do it all the time. No one cares.
One example that comes to my mind is Thr Order of The Stick, which is famous for being a web comic that made a million dollars on kickstarter project for a printed version. It is probably the most popular Dungeond & Dragons based story around and the author does it all the time.
In fact, as far as I remember, the katana is listed in many D&D books (DM Guide, Player's Handbook & Monster Manual come to mind) at least since its second edition (Advanced Dungeons and Dragons).