Why does Song speak in Italian at the end of Act 1?

Hwang is throwing back to Gallimard's earlier parallel of Madame Butterfly with Gallimard's own story. Song picks up on the fact that Gallimard wants her to play the part of his inferior and loyal lotus blossom, and she seems ready and happy to fulfill that role. So, she agrees to be Gallimard's Butterfly, and further more plays up this fantasy by quoting Butterfly and playing the part for her lover.