I hope this solves your problem.
I Using Japanese annotation sources works a lot, so every time I open the source file, I have to do “Encoding/Character Set/ShiftJIS”. Can we make Notepad detect it automatically? I have tried many options in the settings/preferences… but didn’t find anything useful, can we make it through a plugin or rebuild NPP with some additional sources?
This is because the font used by the default theme (Settings -> Style Configurator -> Select Theme) is set to Courier New and is not compatible with Shift-JIS. If you change Change the font to JP-friendly (e.g. MSゴシック), and your file should display JP text correctly without changing preferences. You can also change to other themes that use more JP-friendly fonts. Themes with no font set (left blank) It should also be useful.
I hope this solves your problem.