Use this
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After install you'll have a right-click choice for 'locale emulator' and run the file with simulated japanese system, no need to actually change anything. Take note to chose the right version, Win10 needs the 2.4.x.x one.
It's safe as far as i can say, some AV's might give a warning as the way this tool works interfaces in ways some viruses also might, but its source code is on GitHub, sooo ...
It doesn't translate though, while the dialogues are, the options and so on are still not in english, but that's not gonna be fixed by changing locale either.