A full fledged...what are you talking about....

Im not trying to get mIRC to run my email client and my MMO games Im using it to chat.

Maybe you dont understand what Im saying here.

Spellchecker, as an option, that uses external dictionary files, which can point out typos in a non-intrusive manner (underlinehighlight...change colour).

It wouldnt make any large increase in files size. Dictionary files are available so all you have to do is use one from your native tongue. If you want a word in the dictionary file you just add it.

It wouldnt interfere with any typos or l33t speak you intentionally provided because it wouldnt change text unless told towouldnt even beep or pause you with a message box.

You seem to be very heated over the addition of something that could only improve the chatting experience, mind if I ask why?