Select them from where? The installation dialog?

Select from the mIRC options. The list of languages could be presented in a list box that allows for multiple items to be selected. Never said anything about the mIRC installer...

You expect that to list 200+ languages and for a user to find their specific one?

Yes. Why not?
People do it for their word processors, people do it on websites which ask your geographical location when you register for whatever service that site provide and there's probably numerous other places too where this happens. Why should it be such a problem for mIRC users to pick their languages from an alphabetically-sorted list? Are people so inept that they can't handle that?

Just disable it? But what about people who WANT the feature, but don't want it to affect certain words but can't be bothered to add 100 words to the dictionary? Just disabling the dictionary if it were implemented is hardly an option. I don't see many threads on this board saying 'Oh well, you're having difficulties with that option so just switch it off'.

Well what possibilities are there? Let's see:
1) Add the word to the dictionary via a text field in the mIRC options desiged for this purpose.
2) Don't add the word to the dictionary and live with the fact the occasional word gets turned red in your text box before you send out the line of text. If this is the desired choice, don't moan about it because you're too lazy to amend your dictionary via the mIRC options.
3) Disable the spell checker.

Words in different languages could quite easily conflict. 'Salut' in French may be interpreted by an English dictionary as 'Salute'. How should the dictionaries react? Is the person meaning to say 'Salut' in French of have they spelt 'Salute' incorrectly? Perhaps they should just disable the feature

Since the word is in one of the dictionaries, it wouldn't respond. In such a sitution, you'd have to use your own personal judgement.

I would say most people couldn't care less. If it's intelligible then there isn't really an issue, is there? If it's not intelligible chances are a spell checker is not going to help the person.

But the whole point of the spell checker is to check YOUR spelling, not other peoples. Why would you yourself be writing unintelligable words?!

Why should such a major feature be incorporated for what I would say is the minority of IRCers who are so pedantic they think 'pls' is the end of TEH world.

Another advantage to a spell checker (which my friend brought up) is that it possibly could be advantageous to those who may be learning another language and would like to be able to communicate with it effectively. And please don't snipe at me. I'm not pedantic and I certainly don't see 'plz' as the end of the world. I just prefer my own personal text to be perfectly legible.

Read the last 2 paragraphs in my last post which are still the icing on the cake for me, even if every problem above is answered to its full extent.

You've not really outlined any serious problems that could not be answered with a little bit of serious thought. I still see no good reason why this cannot be added besides the "it's too IM-like" complaint.