SO! I tried using this japanese font i found on google, and set my font to it using my plugin.
All english letters will be changed by the font but still show as english, as my system is set to that language. even in windows, the .ttf file shows as an english font. However, when i use this multilingual keyboard and put some japanese text in there it does show up properly. I dont quite know what it would be like on a japanese native system, but it looks to me like it works just fine without needing to make any changes.
Try the plugin out and see how it works for you! I tested it in 5.5.2, so i dont know if it works in earlier versions.