David Peterson, creator of the Dothraki language, has written an article for CNN revealing that the producers of Game of Thrones hired him to create two dialects of Valyrian, the ancient language of Valyria: High Valyrian and Low Valyrian. Though it’s light on specific details, it gives a great sense of the process of differentiating one version from another.
Having watched the first four episodes, we can say categorically that Peterson’s done a fantastic job. It sounds quite remarkable in the mouths of the actors who speak it at length.