At (nearly) 13 months old, Nate seems to understand most of what I say to him. He doesn't understand most of what is said, but most of the things I tell him (do you want to eat? give me that. don't eat that! take that out of your mouth. are you ready for a bath? bring me a book. it's time for Elmo...). Also, he has a handful of words he can "say."
He says Mama and Daddy regularly. He has said Elmo twice. He signs milk, more, eat, and bath (that one's not real sign language, it's his own sign he made up for bath - pretending to splash the water with his hands).
He can tell you what a snake says (ssss), a monkey says (AAAAAH!), and a baby says (Mama [in a high pitched voice]). I'm trying to teach him Daddy says pffffft! (raspberry sound). Is that rude?
He also has some words of his own invention. Aiyiyiyiyi means he is chewing on something, wants to chew on something, or wants to put something in your mouth for you to chew on. It's the chewing sound. He frequently says yeesh. Sometimes it means he wants a drink. Sometimes he says it when I ask him a question - any question. Like, "Yeesh, Mom, get off my back!"
He does not repeat things that we say to him, or did not until yesterday (unless it's raspberries or sssssss or screaming). In Swahili, haya means ok. I will often sit down on the floor with Nate and say haya, as in, "Ok, what are we going to do now?" Yesterday, when I said, "Haya," Nate looked at me, grinned and said, "Yaya." So I said it again, and he responded with "Haya." We said haya to each other back and forth a few times. This morning, I was telling Rodgers about it, and Nate again said haya with me. I don't know that it has meaning for him, but it's a sound he hears from us, which he figured out how to mimmick.
Regardless, I officially declare Nate bilingual.