High Vinthish is for all intents and purposes, a dead language, now taught only to the intellectual elite. It was widespread before the time of The Great Purge to facilitate communication among all nations; known as the "Language of the Ancients". Special care was always taken to keep it pristine, meaning it was never allowed to evolve naturally as all languages tend to do.
It is not completely dead, as the inhabitants of Overvale (secluded from the rest of the world) still use it. Because of this, Dreya is able to communicate with a limited number of people when she leaves Overvale (Dawn of the Last Bullet).