You need to have your switch modded first, then put the files into romfs folder, the same way you usually do when you mod a switch game.
There's a few typo here and there during the start of the chapter when you first enter HolySee's conclave, and some quests still use the localized name (one still has Poppi instead of Hana) but none too serious. The biggest downside of using this mod is that you might get confused when you're trying to look up a guide online to see where to go, which item to get, who to talk to, because every online english guide will refer to the botched job localizer's made up name which is how I learned about the scale of Treehouse's retardation. They changed almost every town/island name like Superbia and Invidia to something nonsensical. I found out later those names apparently reflect the 7 sins in latin which is probably why they deem it necessary to change so as not to offend the prudish mutts but that still isn't a good excuse, I play Japanese game to enjoy messages from Japanese not to immerse myself in western religious nonsense.
And on being offensive, I find Nia/Niyah's voice offending me greatly. Apparently the voice actor NoA hired had too much woman pride to act like an otaku's cat girl which is her original personality. It's star ocean 4 lymle all over again.