Alright, so the idea's pretty simple, something like a translator mod (which I've seen out there before) but with the ability to add custom languages, so for example, if one were to play on a roleplay server where the had custom languages, you could create your "dictionary" of words/translations, and then, if you enable the mod, whenever you send a message, it would translate it using this custom dictionary by replacing your words for whatever they would be in the custom language according to the dictionary you gave it. So if for example I sent the message "Hello, my name is elfemtog" and I had, let's say, an orcish dictionary enabled, it would check the dictionary (Which could be a simple text document with a list of words-translations) for each word in my message, and switch them out for their translations in black speech before sending my message as "Ug, mi nume bi elfemtog" (this is my bad LotC orcish ^.^) This would make roleplaying the use of strange languages faster, instead of having to translate manually from english to the language that your character speaks.
Some features that I think the mod should have would be:
Multiple language support.
A hotkey for disabling/enabling the translator.
A menu to select the dictionary that you want to translate your messages into.
There's already lots of translation plugins for servers. All you'd need is to write the translation and have the server owner add it to the plugin's language folder. Good luck getting anyone to rewrite the dictionary in a fantasy language, though...