Our two Greyhawk articles this week deal with language. One of the best articles I've seen on the subject is from Tim Bugler (with Ken Barns) entitled Languages of the Flanaess. This is an extremely accessible treatment, and gives an idea of the relative, suggested use and popularity of each tongue.
A slightly different treatment can be found at Bastard Greyhawk. This second article tweaks things a bit, but is a nice treatment of languages in Greyhawk with a few changes. I do wish the font choice was a bit easier on the eyes.