That sounds worth asking around about. I had heard of LayeredFS before, but I had always assumed it was a much more deep-devel-level thing, I guess?
Correct me if I'm wrong but for this to happen a mere ROM dump would not be enough? I'd need a dump that has been annotated with which files correspond to which functions so I can know what parts to carry over for trying the menu translations?
Ok, I'll explain briefly.
LayeredFS is a Sys-Module already integrated in Atmosphere. What it does is redirect the reading of the resources of a specific game to yours, something like modifying the ROM without needing to unpack or inject it directly to it or the same as when you load Textures externally in the emulators to improve or change its artistic style , although this does it with all the integral resources of the ROM. Previously it was used to load games over others, before there was the installation of backups, but it ended up being used more to modify aspects of the games, such as character models, add new things, etc.
LayeredFS will read the resources stored in "atmosphere / Content / [APPID] / RomFs / [ContentGame]" this is the default path, previously it was not in "content" but in "tittle", but that changed with atmosphere recently.
So knowing this, you just need to know how the game is structured. You can use the Switch ToolBox which allows you to internally explore the ROM (if you have the Switch keys and the title key). Everything that contains RomFs can be modified by LayeredFs if it follows the structuring pattern of folders and files.
Now you can compare different versions of a title from each Region. In the case of MHGU Ver. Japonesa y Americana. Verify that it contains the same structure. Check if the language pack is separate and check if it can be replaced by the American one. It may not be feasible because there are many modifications made when localizing the games, since the American version does not only contain one language, but several.
The only disadvantage of this is that if the resources are packed or encrypted within the game, you will have to modify these files directly with the changes you want so that the game does not crash.
https://github.com/KillzXGaming/Switch-Toolbox
https://gbatemp.net/threads/extract-nsp-nca-xci-update-all-in-one-tool-for-layeredfs.511156/
https://github.com/nadrino/SimpleModManager