I'll say it from the get-go that I have no idea of how the translation process looks like for this game (like, does it translates ALL of the text, or just the ingame text, leaving the image names and code parts in japanese?) or even MZ engine in general, but based purely on the history of this circle's previous games they've been dealing in the past - "The Evil Guards of the Merchant City" and "The Dead End" - both of these games had very similar additional DLCs (purely cosmetical stuff with no new gameplay content) released later on and, as I recall, we had them as is, without any additional translations needed.
Those were localized by kagura games. And by the time Kagura localized those games, the games themselves already had said DLCs.
Its diferent than adding new stuff to a already existing localization.
And the process is very simple. in the data folder u have files, in those files u have the dialogue , imagecalls(which summon specific images at specific times).
These "cosmetic updates" take a lot more work since the imagecalls for said new images will need to be added to the data files.
if you prefer a simple example:
Scene 1 in kg version haz the girl showing up with short hair (that means in the files the imagecall is "shorthair.png")
Now in the new version that will come with the extra customization scene 1 can have the girl showing up with short or long hair( either imagecall "shorthair.png" or "longhair.png").
Those choices need to be scripted into the data files, else the images arent used.
Most people think "oh its just copy pasting the translation files into the new version". Well it isnt, since that just make the game to back to the version when the translation was created.
As for :
"like, does it translates ALL of the text, or just the ingame text, leaving the image names and code parts in japanese":
KG always changes the images names and "image script calls" of the games they localize to english no matter the game they pick, so it doesnt cause crashes in english systems.
That is why japanese save files arevery rarely usuable with kaguragames versions of games, since they call for images and other scripts with the japanese names.
TLDR: if u wanted to manually add the customization to the new version u would have to carefulyl go trough all the files in the game copying the translation(dialogue only) line by line into the new version and hope it worked. But doing that would force ya to use japanese locale to playing the game.
Doing the reverse aka adding the new customizations to the kg version would require u to translate every single new image file to english then go trough all the game files and add the new imagecall sscripts to the each file(properly) and then translate the image calls there to english.
2nd option is never viable since kg does some really weird edits to the general scripts when localizing games for some reason (like adding lines that cause fontsize to change , or scripts that change font types (look at sister monochrome).
Edit:Reminded myself of another example in monochrome, while in the fan translation theres a set of images called "bukkake" in the kg version those images were renamed to "book".