This doesn't seem like a very feasable thing to implement directly into mirc, the dictionary files alone would be a huge bulk to mirc (or resource hogs in the case of active online communication).
Mirc is used by so many people who speak and type in so many different languages that it would be near impossible for an internal spell checker to stay up to date.
Other problems would arise also, such as users that speak more than one language on irc, in the same channel even.
