Not all characters will show up in the edit box, though once mIRC's fully unicode, that will likely change. However, you can still paste it and it will send to the channel. Whether or not it gets sent correctly to the channel and whether or not it displays correctly on your screen depends on your mIRC settings as well as your font. And whether or not someone else can see it even if it's sent correctly will depend on THEIR settings and font.
Note that language packs aren't required to paste and/or display Japanese characters in mIRC.
