I found an interesting new file posted over at www.worldofwar.net that allows cross faction discussions (if both parties have the add-on). They make no claim as to this being a legal add on or not, so please use caution.
Have trouble understanding the other faction?
This is a simple Add-On for use in World of Warcraft, that allows you to speak to and understand other factions and languages. The extent to which this will work depends entirely on the amount of people that use it: the BabelFish translates any in-game language into an intermediate code (which I playfully refer to as "Babel"), which your console will then translate for you automatically! However, the speaking player must be using the Babelfish's translation script in order for the translation to work properly.
The add-on can be found here.