I premise that I have some experience in translating, but only with pre-configured Tools (my programming level is zero).
Let me explain:
I have these 3 versions: FFX International original, FFX PAL (italian) and FFX International Undub (Japanise dubbing).
I'd want to:
1. replace japanise texts in both International versions with italian texts (so to have eng and ita texts)
2a. putting both eng and jap dubbing into the same ISO if possible adding a selector at the game's boot (see 2b)
2b. merging the 2 ISOs into one, adding a selector at boot, similar to this "one-iso ps1 Crash Trilogy: https://www.youtube.com/watch?v=Z0i9R5fX124
The 2b solution would result in a ISO with double the size, but maybe it'd be easier to do.
in any case it'd be possible to change the dubbing Language at any point of the game (since savegames are perfectly compatibile).
What do you think, it's something feasible??
Btw I explained the best possible scenario, I would be satisfied enough, even just being able to put italian texts into the International version.