Sally is talking with her friend, Don. (S=Sally; D=Don)
S : Tell me, Don. How come you're from Spain but you can't speak Spanish very well?
D : It's not my mother tongue. My father is Spanish and my mother is American. We usually speak English at home.
S : Oh, I see.
D : Your Spanish is probably better than mine.
S : Yes, but you speak English more fluently than I.
D : How come? Aren't you American?
S : Yes, but my mother is Spanish.
D : Don't tell me...
S : That's right. We usually speak Spanish at home.
D : It doesn't matter as long as we can communicate.
S : That's true.