The Treacherous Tongue of secrets and duplicity. It is the language of the Kindly Folk and their trickster-emissaries: cats, rats, foxes, and monkeys. Treat each Rank as a Skill Rank in Cryptologist. A Fluent speaker can choose to be understood only by those they designate (even if they do not speak Puckish).
Learning this Tongue makes the speaker vulnerable to the magic and manipulation of the Kindly Folk.