@Acolytus Well, Shin did learn the language of the world, and is speaking in it, yet it was probably he that messed up.
In chapter 26 we see him using the "maou" for the first time, but by the looks of it, there is no word in that language for "demon lord", so he probably ended up making up a word derived from "magic" and "king", instead of "demon" and "king", because they're the same in japanese.
He probably did so because of a poor understanding of the peculiarities of japanese compared to other languages, as he was never taught the world's language in comparison to japanese.
Overall it's a kinda dumb mistake on Shin's part, and it also shows that the author has a shallow understanding of languages other than japanese.