More than likely the only way mIRC would support spell checking would be to 'borrow' it using COM's from another application, i.e. MSWord, and for this to happen, as one might expect MSWord must be installed.

Failing MSWord being installed mIRC would then need to connect to some dictionary website of some sort to check the words which would be unreasonable to expect.

And, failing that mIRC would need to come packaged with a nice big dictionary which i'm guessing no-ones going to like.