Well, you can't underline words in red in mIRC unless some DLL I haven't heard of can do it. But you could pop something up that listed unknown words, I suppose.
The lag issue is with the methods you'd have to use to check the sentence in mIRC scripting. mIRC scripting just can't check it fast enough, afaik, to prevent some kind of lag if you're a fast typer and type really long lines of text (or paste them). A DLL would be needed to handle it fast enough for it to work well.
However, if you had a hash table filled with your dictionary words and each word was an item in the hash table, you could probably do it without lag by just checking $hget for an exact match. Still, with long lines, you'll have to loop through every word on the line, which can still cause lag. Also, you have the problems of punctuation and other symbols. What do you ignore or remove before checking for an exact match? You could remove all symbols and keep only characters when checking, but that could cause problems if certain words need apostrophes or hyphens or whatever else.
And, I don't know of any dictionary files that are formatted properly to just load into a hash table in the first place.
