Sally is talking with her friend, 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.

mother tongue = mother language = native language
My father is Spanish and my mother is American.  Spanish,American是adj.
or: My father is a Spanish/Spaniard and my mother is an American.  n.
spinach 菠菜
Betty bettered her performance by studying hard.   better vt.提升
He is a fluent speaker of English. = He is a good command of English.
A friend in need is a friend indeed.
Don't tell me. = I don't believe it.   coincidence