Pete, I think that Michael's conversion to english was to further dramatize the murder scene for the audience. It allowed Michael a point of departure after hearing Sollozzo claim to be the hunted one and desire only a truce. Thus, the audience is informed that Michael 's suspicions about Sollozzo's intent are confirmed.

"Generosity. That was my first mistake."
"Experience must be our only guide; reason may mislead us."
"Instagram is Twitter for people who can't read."