Leatha: "Why not?"
Beck: "I don't want to learn Mexican."
Leatha: "Mexican? What do you speak now?"
Beck: "I speak White. Trey [Nesbitt] doesn't speak English, he speaks White. He's black, but not that black."
Leatha: "Beck, you and Trey speak English."
Beck: "I guess I could learn Mexican."
For some context: I think Beck is troubled by stories of his aunt and uncle, who are currently in Central Asia and in the arduous process of learning another language. Turkish, Mexican, English... to him are all foreign languages you have to learn when you go to another place to tell people about Jesus.
He prefers his native tongue...White.
But for the sake of the Mission, he's now open to learning Mexican.