Thanks for the info. In hindsight...that defo would've been the better decision on my part. lol
I managed to work it out somehow (wasn't easy for me unfortunately, but there you have it).
(Just as I suspected, one) cannot (re)pack anything with non-unicode in their filenames (which some of the files from your patch have) in non-JP actual system locale, so had to bite the bullet and switch to JP locale (ouch). Then there was the matter of me trying to preserve the file structure (system2 and scenario) of the patch... which as it turned out, was a bad move. xD
Anyhoo, a good couple of tries later I think I got it right; at least that Anvil scene in the prologue finishes correctly in English now, too - so I think the patch is applied correctly.
Which UI buttons are we talking about? These are still in Japanese for me (maybe these are not the ones you were talking about, though...):
But I'm gonna go out on a limb and say that's because this is the DLsite current(ish) version, and not the worked upon fanTLed one from the OP. And the devs are "still working on it", I guess - as you yourself had said earlier.
What UI buttons was yours supposed to fix (for the one in the OP), btw?
Also, lemme just say it's quite chaotic how they have different versionups for the basegame and the 6 DLCs - can't really figure things out from DLsite, really... I think e.g. the 210407 update of the language addon *should've fixed* the Anvil scene bug (at least it says that that update fixed errors in the en scripts) but alas... Also, the added v210414 update info for 4 of the other DLCs, which I think only is the language-addon's v210414 fixes... We are still probs pretty far away from a definite ENG version, that's for sure...
(Very tangential) side question: how would I go about creating an all-containing 'data' folder from a 'data.xp3' and the six 'patch_*.xp3' files? I mean I can extract them now, sure, but I guess I need to get the files from the separate patch folders (sequentially, and overwrite when needed) to the new data folder; but those patch folders have no folder structures... I mean, hunting down where each and every one of those files' counterparts inside them are in the 'data' folder... Sounds like a real pain...
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EDIT: ahh, I see, for example you fixed up the Gallery menu to be fully in English and look better
EDIT2: and actually, your patch makes the voices play on my end... *thumbsup*