Mutiple locales on system, CJK fallbacks

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:

  1. You can create a ~/.fonts.conf.d/05-language-fallback.conf file to customize the preferred language fallback used by fontconfig.
  2. 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.