Baby (language)

"Baby" was the name the Eleventh Doctor gave to the language spoken by human babies. It was spoken by infants Melody Pond and Alfie Owens. It appeared to be a one-way language, spoken by babies and heard by the recipients. The listener — in these cases, the Doctor — responded in standard English, which the babies seemed to be able to understand.

Judging solely by the Doctor's responses, "Baby" was a complex language that had a complete grammar, a wide vocabulary, and many rich colloquialisms. (DW: A Good Man Goes to War, Closing Time)

Some of the information gleaned by the Doctor from speaking baby included:
 * Infant Melody Pond's penchant for calling Amy Pond, "big milk thing"
 * Alfie Owens' preference for the name, "Stormageddon, Dark Lord of All"
 * The fact that Alfie called Craig Owens and the Doctor, "Not-Mum", and referred to everyone else as "peasants"

The Fourth Doctor also exhibited an ability to speak Baby. When he and Sarah were in Pescaton-terrorised London, they came across an apparently abandoned baby. Unable to get any information from the child, the Doctor said, "He won't talk." Unaware that the Doctor was likely speaking literally, Sarah then responded, "He can't talk." (AG: Doctor Who and the Pescatons)