this is something I've wondered too. I don't own a hacked or exploited system, but I have downloaded every game I own. unfortunately, well over half are the European version. people say the system is region free, which it is, but dlc and saves are region locked. I know saves are at least, cause I once had two poi saves. I think new super mario deluxe can be used in any region so long as you change the content/title id for it. maybe other game saves can do the same. there's probably a way to convert them if they can't be, but as I said, my experience and knowledge are limited on the subject.
might work, might not work.
your savegame contains text, which is written in western characters, which the other gamevariant might not have!
so your save might work, or might not work, or crash the game.
might be able to tell with switch backup manager. it will tell you which languages are supported by the game. in the case of new super mario bros. deluxe, I'm pretty sure a usa save works on a Japanese game and vice versa. it's a universal save, just change title id of it.
Usually if the game was a localized version, the save from the Japanese counterpart should work fine, you probably won't be able to get anything that is offered in the Japanese version over to your region, if it is DLC it would be locked to Japan probably.
would you still need to change the title id? the reason I ask is those two poi saves were seemingly from the same game or at least were in English. I had to guess which save went to usa digitally that is. the cartridge says it's for a different region than my account, so I can't get coins with it. however, it says USA on the cartridge. it's a mystery what the game is.
probably have to change the title id if that poi save debacle is any indication. I accidentally deleted the wrong save, so I had to redownload it from nso. nintendo system saves are usually the easiest to work with so chances are you won't have to do anything but change the folder name. you may have to hex in the title id, but that's only if it doesn't work move on to attempt 2.