Yeah, he begins his response to Sollozzo in Italian, and then when he stumbles over words, ("Come si dice...") he just gives up and goes to English ("What I want...").

If you have immigrant parents, you can understand that he'd stumble a bit over his second language. He knows Italian, and understands it well, but remember, his parents spoke mostly English to him in the house... that has a lot to do with it, I bet.