Android 4.1 has a better glyph support for Japanese users (renders Japanese-specific versions of glyphs WHEN system language is set to Japanese).
I use a lot Japanese (reading / writing) and what see when system language is NOT set to Japanese is the Chinese glyphs characters instead of Japanese ones. Often those are the same as the Japanese glyph but in some cases they are not.
But if I set the system language to Japanese the glyphs are the good ones. So, it's not so convenient to set system language to Japanese every time I need to read or write in Japanese !
It looks to be done with a fallback fonts list that gets switched with the UI locale. Inside of /system/etc/ there are two XML files: fallback_fonts and fallback_fonts-ja. In the ja one the order of MTLmr3m.ttf font is placed before DroidSansFallback.ttf. The MTLmr3m.ttf file is the Motoya Japanese font and the DroidSansFallback.ttf is the fallback font that has the simplified Chinese glyphs.
The fix to allow Japanese instead of Simplified Chinese with a non-Japanese UI would be to copy the fallback_fonts-ja file to fallback_fonts. Before doing that it would be a good idea to copy fallback_fonts to something else like fallback_fonts.backup so that you could restore it later. However, before doing any of this you would also need to root your device since the /system directory can't be edited otherwise.
I even tried to edit the fallback_fonts file to put MTLmr3m.ttf font before DroidSansFallback.ttf. But in any case, once I reboot my Nexus 7, the Japanese font are not read anymore (juste leaving blank).
Does anyone have a solution to use the MTLmr3m.ttf file (Japanese font) instead of DroidSansFallback.ttf without setting the system language to Japanese ?
Thank you !