"Religion" isn't a word in "most languages". "Religion" in English and the closely-related terms in closely-related languages may mean (or at least strongly imply) something like that (though with a god or God, specifically, not merely "a supernatural entity") simply because of the dominant belief systems to which the terms applied in the West (mostly, varieties of Near Eastern Monotheism), but even so they are frequently applied to systems from outside which don't have an "ordered belief in a supernatural entity", such as Confucianism.

> If it does become a synonym, then we'll simply need a NEW word to distinguish between "belief in supernatural entity" vs "no belief in a supernatural entity"

With, again, the caveat that it is more specific (referring to one or more gods, not merely any supernatural entities), we already have such a word, adopted specifically to focus in on that aspect and not the other elements of belief and practice which "religion" typically includes and may sometimes be used to refer to without an identifiable god anywhere in sight: "theism".