Use fallback Unicode font for missing characters in subtitles font.
Japanese kanji, amongst other exotic characters in subtitles, do not show in subtitles with default font or font that does not support those characters is being used.
Use fall-back Unicode font for missing characters in subtitles font. A default fall-back Unicode font should be in the preferences and the user should be able to change it. A smart default setting is not easy and depends of what is installed on the system. Nevertheless some smart Unicode font detection approaches could be implemented.
Also notify the user that some characters are not supported in the subtitle font being used. This should notification which can be blocked from repeated popping up. Also it should give the user some information on how to change the default font to prevent this.