Also keep in mind that mIRC's used by people all around the world and including dictionaries for multiple languages would be pointless, while just including an English one would be a bad idea as well.
You can just use the same system like opera/firefox do. Aspell...
With firefox you can easy download a dictionary for your own language as a addon.
I think a spell checker can not be done by a script because scripts are to slow. It need to be hardcoded. Also no good idea to reinvert the wheel because there exist already toolkits which work proven very well.
