I have some interest in Esperanto (I wonder if JR has seen this video?), and have been wanting to incorporate it a little into my game as an example of what Common looks and sounds like. Aside from whatever flavor it could add, it might be helpful in actually picking it up a bit more... I imagine a game entirely conducted in it would be quite surreal. The Common tongue in D&D seems similar to Esperanto (or what it wants to be) in the sense that it's a universal language in most settings , albeit a natural one. Having individual human languages or Common dialects might add some twists to game play and be justified by there being human inhabited but extremely remote or culturally backward locales to explore. It might be a great way to incorporate a quick game of charades into a session!
Marc Okrand created an artificial language of the Klingons from Star Trek - I think it’d be cool if there was something similar for Orcs or Goblins. I was looking around for a translator of some sort (I wanted to add some "real" goblinoid into a room description) and didn't find much, but stumbled on a number of fascinating articles on Helge Kåre Fauskanger’s site having to do with Tolkien and the languages of Middle Earth. Especially interesting was an article on “Orkish and the Black Speech”, which might contain a "reason" if you will for why nobody's devised a dictionary for it.
It seems a shame there's not any true or complete artificial language (AFAIK) for any of the various monster species in our favorite game. Linguistics student needing a project? Get on this! You’ll be famous.
Update: Fauskanger mentions that there appears to be modern usage of Orkish and bat has brought to attention the band "Za Frumi" which led me to an online tutorial of a home grown dialect.