Maggie first talks at the end of "Lisa's first word" Homer takes her to bed, says goodnight and closes the door. It is then that she says, "Daddy" before falling asleep. It's my favourite Maggie Simpson moment, cause it's so cute!!!!!
Maggie has already talked. At least in two episodes. In one episode she says daddy. and in a 20th season episode "Four Great Women and a Manicure", Maggie talks quite a lot at the end.