The True Tongue is the language of the children of the forest, in which they sing the songs of the earth. The name the children call themselves in the True Tongue is those who sing the song of earth.
The language was said either to have originated or drawn inspiration from the sounds of the natural world that the children of the forest were surrounded by. When speaking the True Tongue, the children are described as sounding like the wind through leaves, rain on water, or the sound of stones in a brook. Their songs were said to sound as beautiful as they were, with their voices as pure as winter air.
In addition to the children of the forest, it is possible for ravens to speak the True Tongue, and they can be made to repeat words in it by the children in order to send messages over great distances, though most maesters dispute this. Humans are unable to speak the True Tongue, but an ancient ballad tells the tale of how Brandon the Builder was able to learn to understand the language while seeking the children's help to build the Wall. This account, recorded in the book Winter's Kings, is one of the only fragments thought to have survived from the time of the First Men that records what the songs of the children were about.
The children called Dorne the "Empty Land". Thus the word "Dorne" may have its origin in the True Tongue or it may be a derivitive made up by the First Men who were unable to speak the True Tongue properly. However it may also be entirely unrelated to the True Tongue and instead be a word in the Old Tongue of the First Men.
A Dance with Dragons
Bran Stark hears Coldhands murmur a blessing in a strange tongue when the ranger slits the throat of his fallen elk. Within the cave of the three-eyed crow, Bran hears Leaf and other children of the forest sing in the True Tongue.