If memory serves correctly, fixedsys is the default font for mIRC, but you can choose the font you want mIRC to use, in any mIRC window just type /font
The fonts which mIRC can display will be listed in the fonts dialog. As for which one would be best for Japanese, i'm sorry but i don't know. Others have posted here about using Japanese, so hopefully someone will have suggestions for you. You could try going to a channel where there are likely to be others using Japanese and asking them what font(s) they use. To get a list of some channels, in any mIRC window after you are connected to a server type /list *japan*