His Sicilian would naturally have gotten better while living in Sicily after he kills Solazzo.
It was still unrefined then, at least at the point where he needed Fabrizio to translate to Appolonia's father. He wasn't shown as fluent until Part II.
I think that might have been a clever power play on Michael's part. He orders Fab to translate, but obviously shows that he understands what Sr. Vitelli is saying in reply. Could be Michael showing Sr. V. that he's in charge--and that he knows enough Sicilian not to be fooled.
Interesting theory. It does show his superior position over Fabrizio. I'll have to rewatch the scene to see how much understanding Michael demonstrates.
