Isn't that usually just replacing japanese text with english in the game files? It worked that way when I tried translating games from english to my language, or modded games. Text was usually separate from source code, in assets, and changing it was as simple as just deleting the original line and writing the translation there.Then program it into the game.
I'm not tech savvy in any sense of the word, but the most common issue are character and memory limits when it comes to JP->ENG translations.
Most typical issue being that name fields only allow for something like 8 characters, which works out fine with kanji, but is next to nothing in English. Hence why demon types sound so unintuitive in SMT, character limit resulted to "bird of ill omen" to be changed to "raptor".
Then there is memory fuckery. The guy doing the hacking for Sengoku talked about it a bit in 4chan. The game had a bug where it seemed to always crash after you played for a while. Turns out that it purged some values in the memory after their allocated spaces filled, which of course happens faster with latin letters since they hold less information in the symbolic sense compared to kanji.