From the general forum topic.

Quote Originally Posted by Corvinoobus View Post
Hey y’all,

It may surprise some of you to learn that a great deal of official terminology is determined through a collaborative effort. The Japanese lore team frequently consults with the localization teams, asking them to provide suggestions, which are then localized into JP.

As some posters have observed, “glamour” has an archaic meaning of “magical illusion,” and we have used it in this fashion since 1.0. Given the lore behind this new system, it was an obvious choice for the English version.

However, “glamour” wasn’t an ideal word for the Japanese version. It doesn’t possess the desired archaic connotation, and when written in katakana, it can be misread as “grammar.”

Therefore, the item name “glamour prism” (グラマープリズム) was localized into “ミラージュプリズム” (mirage prism), which would more effectively convey the intended meaning to Japanese players.

I hope this clears up any lingering confusion as to why the names are different!
In short, it was originally meant to be Glamours in Japanese as well, but was changed to avoid confusion for Japanese players.