They really shouldn't.
If they were to do so, then the game would be incompletely translated. Inviting more trouble.
You see it often with MTL games that some stuff is included in the RPGM save files. This ranging from costume names to item names and amounts and stuff like that.
Often changing stuff in a translation (be it MTL or otherwise) can completely break the saves of that game (or be totally undone by using an existing save that already has the stuff from before it was fixed).
RPGM uses a proprietary build format for their save files and none of that has, as of yet, been editable.
Save editors just manage to find some basic stuff like switches or stat values here and there, but none of the advanced stuff like that as far as I've seen.
I don't think even pro DEV's bother trying to hack the format either, as I've seen often that a 'new version' of a game in development 'breaks' the save files of an earlier version.
That's kind of the trade off you make by using only an existing editor/Engine that pretty much 'gives' you the gameplay OOB, rather than making something from scratch in Unity/Unreal/Whatever.
Same with dynamic scripting with Japanese tags and such. It would make sense that a full, professional translation goes and translates all of that rather than half butting it, leaving the Japanese tags and not touching the code.
Though I've not yet checked any Kagura releases for this, unpacking them and checking for this.
Also ... they don't make patches for Japanese games. They release and sell localized games.
So they literally can't 'break' anything for anyone else.
Whether or not you already pirated the Japanese games before hand really shouldn't be any of their concern.