mIRC is not used only by English users. Adding dictionary support would require adding support for many languages. This is likely not going to happen, though I can't speak for what Khaled decides to do. It has been mentioned in the past, so he knows of the small number of users who want it (or at least only a small number are vocal about it) who want it. If he feels like it would be worth adding, then it's mostly likely already on his list of things to do.
Scripts that offer dictionary support usually offer it one of two ways. Either they include a dictionary file (or files) for one or more languages that can be used. These can greatly increase the script file size, but can work fairly well. The other option is to connect to a website and use it to determine if words are spelled correctly. This requires a lot of web connecting to check everything typed and really isn't very good for performance as it can be slow depending on the site you're using. Depending on the site, this can allow access to any number of languages, however. It also doesn't need a dictionary file for each language, so the file size of the script is much smaller. So there is good and bad.
In any case, chat is like IM -- in that it doesn't need to have correct spelling. Few people care about it in chat.
