Enochian

A magical language which is thought to be older than Sanskrit, with a sound grammatical and syntactical basis. It resembles Arabic in some sounds and Hebrew and Latin in others. It first appeared in print in 1659 in a biography of John Dee, the famous Sixteenth century seer and astrologer. Later used by Aleister Crowley in his spells and Anton Szandor LaVey in his The Satanic Bible.

Enochian is said to be the language that angels speak, and that with this language it becomes possible to summon very powerful being indeed. Ceremonial magicians swear by the potency of it, and imply that even if you don't believe in angels, it doesn't matter since the angels believe in you.
The language is a tongue twister and few words are known. That means you probably won't be able to use it to hold a conversation. Instead Enochian is used to 'call' the intended being by name, and then the conversation takes place in a language of the magicians preference. That is provided the called being wants to chat with you.

Enochia is a lot more than a language, namely a whole magickal system. There's screenfulls upon screenfulls written about it, and hope to node the entire thing out some day, so this node will probably be updated at a future date.