Nope, Middle English. A lot of the other Arthur stuff is in French--Chretien de Troyes, the prose Lancelot, and at least two other works with similar titles, but Malory wrote in English. He was supposedly in prison at the time he wrote it. :P

Although, technically, most of the modern editions, including the Penguin I have, are "translated" in the sense that the Middle English is fixed up with modern spelling--although the edition I have otherwise leaves the syntax as it was.