I work alot with Japanese comment source so each times I open a source file, i must do "encoding / character set / ShiftJIS". Can we make Notepad++ auto-detect it ? I've tried alot with options in Settings/Preferences... but didn't find anything helpful, could we make it through a plugin or rebuild NPP with some additional source?
This is because the font used by the default theme (Settings -> Style Configurator -> Select theme) is set to Courier New and does not play well with Shift-JIS. If you change your font to something more JP friendly (e.g. MS ゴシック), your file should display JP text correctly without having to change the preferences. You can also change to a different theme which uses a more JP friendly font. Themes where the font is not set (left blank) should work too.
Hope this solves your issue.