With what you know via his trivia, apparently he learned Infernal during the same drunken bender which resulted in him being a pirate. His Infernal isn't flawless as any native speaker can tell he's somewhat amateurish, but he has a surprisingly strong enough grasp to understand it even if his own fluency isn't great. He chalks it up to being an accountant, as it was a common joke among his fellow workers that their bosses are actually devils.