How come the Godfather never spoke Italian/Sicilia

In the first Godfather Vito arrived in America, but throughout the part showing his adult life he never uttered a word in Sicilian/Italian?

Was it because Brando could not do it?

In Godfather 2, which is my favourite, much of it was Sicilian/Italian.

Not true! When Vito's about to leave his city office he tells Fredo Andiamo. Then he wishes Dano Merry Christmas in Italian as he helps him put on his coat. Also, at the wedding he introduces Barzini to his sons in Italian and says Bene to the Nazorine as he leaves Vito's home office. He also instructs Tom to not give an assignment to our Paisan and he tells Fredo aspette as they leave his city office. Of course, the highest profile Italian was when he called Johnny a finocchio

Brando spoke Italian twice at Connie's wedding: When he welcomed Barzini to his home, and when he told the photographer not to take the family photo without Michael.

Boom. He doesn't converse with anyone but speaks several lines.

Brando was a bloated idiot who could barely remember his lines in English. How would anyone be able to teach him to speak a second language ?