Merlin, being a Dragonlord, has known how to speak in this language since commanding Kilgharrah for the first time (The Last Dragonlord). His father, Balinor, also spoke this language before Merlin, as did all the Dragonlords. It is unknown if Kilgharrah and Aithusa actually communicate in this language or if it is only a language that Dragonlords speak in order to command the Dragons. Interestingly though, the wise creatures can also speak English and Kilgharrah is seen communicating in this language with Merlin, Gaius and Lancelot. Although Wyverns understand the Dragon Language and, like the Dragons, obey commands spoken in it by a Dragonlord, the feral, animalistic nature of the Wyverns makes them unable to speak it or any other known language.