Welcome to the forum!
The big problem here is that you're using a new Linux that no-one will know anything about. Handy is Debian-based and I just happen to have another Debian derivative installed for reviewing, so I've done some experiments.
Firstly, you need to run
Make sure that both the English and Japanese locales are selected and then set the default to NONE.
The next step is to set the user locale for each user, and this is another big problem. Arch says you should create a file .config/locale.conf and specify the locale in that:
You may find there's a file .config/user-dirs.locale — if so, try altering that. Another possible place is .dmrc.
I couldn't get any of these to work, but Handy might be different. Personally, I'd never use a Debian-based distro when anything slightly out of the ordinary needs doing: the documentation is so often missing or incomprehensible.