High Speech was once the formal language of All-world.
For a time, Roland Deschain and Marten Broadcloak were the only people conversant in High Speech (though Tull-native Nort acquires the ability to speak the language as a result of his interaction with Broadcloak).
The words on the ghostwood doors that Roland encountered on the beach of the Western Sea were written in High Speech.
Deschain used the formal language with Susannah Holmes, Eddie Dean and Jake Chambers after he drew them into All-world.