Personally, I have 2 copies of the game.
Translated one when I want to see most of the game, untranslated one for the new stuff (with Mtool).
If some files are compatible, that'd be great, but it's a genuinely difficult task to figure that sort of thing out, and it isn't worth the trial and error imho.
I don't suppose you're willing to post the older translated copy then?
The translation post is just the translation not the game, and the post from earlier in the thread with the game isn't a valid link anymore, so I only have access to the newer versions of the game.
I'll make do with MTool if I have to, but if a better translation exists for part of the game I'd definitely prefer to use it.
I don't think there is any way to download older versions from DLsite.
Edit: As a heads up for anyone curious, I did start playing the game with the old translation dropped into the newest version of the game, and have yet to encounter errors. I've only got 2 dungeons complete right now, so there might be some problems when I get near the end of the old content and the beginning of the new.
That's basically when you'd have to switch to Mtool anyway, so I don't see that as a major issue unless it somehow corrupts the save file so I can't swap versions. That seems rather unlikely though, so while it's a shame we won't get a proper translation for the newest content, the good translation should still be usable with the newest version of the game.
You'll just have to keep 2 copies of the game and swap your save over the the clean version and switch to MTool when the time comes.