Ciao a tutti! I live in Switzerland and my mother language is italian...
The correct pronunciation in italian is COR-LEON-E, just like Calo and Fabrizio (The sicilian bodyguards) pronounce it. Also Bonasera pronounces it correct. (When he says: "For Justice, we must go to Don Corleone").

In English it becomes Core-lee-own.

When Senator Geary pronounces it correct (like a true Italian), it sounds very forced and it really sounds like an insult for an italian! In fact Geary do it out of contempt!
He only wants to "tease" Michael Corleone and italian people.