Keller

Demons speak demonic language, right? Except, the thing is, demonic isn't really a language; it's outside language. It's the ability to understand anything anyone says to you: the meaning behind the words without understanding the words themselves. And to make yourself understood by anyone in a similar way.

Now you've that in mind you'll understand what an anomaly Dm Keller is when he sets out to learn English. Even more outlandishly, he succeeds, and starts learning more. Earth's selection exhausted, he begins studying languages from the other worlds connected to Hades.

Spoken and written language might seem a useless skill in a species that can understand any half-expressed thought, but Keller enjoys it.