Or Klingon or Elfish from LOTR, those are fully developed if i'm not mistaken, at least Klingon is, and you will find more people who you could converse with in these languages, if you find the right places.
Plus they're true languages with grammar rules and consistency, rather than a cypher like the Dragon language seems to be designed as. That's perfectly good for its intended use in this instance, and I should be able to suspend my disbelief reasonably enough with it, but when it comes to learning a language, I personally would rather have something with a little more meat to it.