I see. At the moment there is no UI to set system wide display language preference order.
The locale should already exist though, you can confirm that by running
locale -a|grep ja_JP
There is another thread about the same topic and there are some tips:
- You can create a
~/.fonts.conf.d/05-language-fallback.conf
file to customize the preferred language fallback used by fontconfig. - Or you can set the
PANGO_LANGUAGE=ja
environment variable in a file~/.config/environment.d/pango-language.conf
to control how most GTK applications will render the glyphs.