Pretty sure there is no way to get this through lore wise. There are “Samurai” dudes in warhammer but they are from cathay (or was it nippon, not 100% sure), but thats on the other end of the world, also they aren’t elves.
Japan has always been what English-speaking nations called it, but in Japanese it’s Nippon, if I remember correctly. That’s a very normal thing, Germany doesn’t call itself that in German, Ireland doesn’t call itself Ireland in Irish in Gaeilge, and so on.
As for Kerillian; Elves have never been seen using katanas in Warhammer lore, though some of their weapons are shaped superficially like them; some of the skins for the Elven Great Sword have a single-edged curved blade, for example.
To add to this, amusingly Germany is referred to as Allemagne/Alemania or some other variation thereof in many romance languages, despite the Alemanni having largely settled in Alsace, Swabia and Switzerland. It makes sense from a historic perspective as these regions border regions where the romance languages evolved, but it doesn’t make much sense to call the the whole of Germany that.