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 complete grammar and a wide vocabulary, It used many colloquialisms. (DW: A Good Man Goes to War, Closing Time)