I don’t use languages other than English, and I use a Dark Theme. You would be able to do this already like this:
1) Save your current Config/Theme to Dark-EN
2) Change the font for the current Config/Theme to your Japanese font of choice. Save the theme to Dark-JA.
When you want to switch to an English document, choose the Dark-EN theme. When you want to switch to a Japanese Doc, choose the Dark-Ja theme.
What might be nice though, is the ability to change Themes from the Drop down menu’s directly. E.g.
Another Option, Further. Duplicate a given language’s highlighter and Name it LANGUAGE-NAME-JA, do the customizations as noted above.
Now you can toggle between something like “C” and “C-JA” directly from the “Tools/Select Configuration” menu.