mIRC stores your quit msg in its mirc.ini (section [text], item "quit") which as far as I know is saved in ANSI format.
Quick suggestion to bypass this issue (maybe there's a more nifty sollution):
1) in mIRC, at tools: options: messages: "quit message" put the following command: $read(quitmsg.txt,1)
2) Open the directory where your mirc.ini is located. To get the right directory, simply type in any mIRC window: //echo -a $mircdir
3) Create a file "quitmsg.txt" in that directory. type/paste your japanese quit message in this file, and save it (make sure you save in "unicode" or "utf-8" format (maybe try both)).
