This can easily be fixed by installing and inserting the Japanese version of DroidSans in the font fallback order, which I've been doing manually for a while but is a bit of a hassle if you like to flash or update roms as it will involve doing just that every single time. Hence I've made a flashable zip for those who are lazy like me or don't know how to/are reluctant to edit the files themselves.
Update: Font handling and default fonts have slightly changed with Lollipop, so here is a new flashable zip for those versions going forward (NotoSans fallback). What this does is change the fallback order for both fonts.xml and fallback_fonts.xml, as well as while we're at it install a complete version of Google's Japanese NotoSans font (the one in system/fonts by default is missing kana, leading to inconsistent rendering).
Should work on most devices and ROMs.
Download & flash DroidSansFallback for Android 4.4/Kitkat
Download & flash NotoSansFallback for Android 5.x/Lollipop