A well-educated speaker of Hindi (a modern descendant of Sanskrit) who doesn't have any special training in Buddhist (or Hindu) liturgy would probably be able to more or less understand the following short mantra. (As far as I can tell, this particular one isn't actually used outside of Shingon, but that shouldn't matter for the purposes of this discussion.)
Namah samanta-buddhanam bhah
The Japanese equivalent of this appears to be:
Would an equally-educated speaker of Japanese without any special training in Buddhist liturgy be able to understand that mantra (or any such mantra; no need to focus on this particular one)? Or is all this Sanskrit-derived Buddhist stuff more in the realm of a foreign language which one would need to learn separately?