mIRC already supports Eastern languages. In order to use Eastern encodings, however, you have to turn Unicode support off and run mIRC in a locale appropriate to that country, either by changing your system settings or using Microsoft's AppLocale utility. There's also a special conversion mode for the Japanese ISO-2022-JP encoding, called SJIS/JIS conversion. This will often be a necessary step, since Unicode hasn't really caught on in Asia yet.

Good luck, whatever you decide to do.