Ventus stressed the importance of being able to speak and understand the Balaur language in the Abyss. He said that Kunberunerk, the president of the shipyard in Sanctum, had a potion that you could drink to learn the Balaur language quickly. So you went to see Kunberunerk and completed a mission for him so you could earn the potion. You drank the potion and learned the Balaur language this way.