If all the tests turn out okay, I wouldn't worry too much about it.
According to relatives, I didn't talk until I was three or four years old. I made the normal simple sounds, but my mom tells me that rather than talking I communicated with my own made up sign language (I would tap my thumb and forefinger together to indicate I was thirsty, etc). Nobody worried about it because I seemed to understand everything fine and found my own way of "speaking."
When I got to kindergarten I still wasn't speaking much and I did the all normal evaluations. Turns out, I was developmentally AHEAD of the other kids - and stayed that way. I understood and processed info at a 10 year old's level, and could speak perfectly when compelled to (by M&Ms!). They suspected I didn't talk because I was just absorbing and learning. Go figure.
Sometimes kids just have nothing to say. I was one of them, and I turned out pretty okay.
