If Biaggo is Italian...

that didn't make sense to me either...his name is italian..he said something during the movie about an italian demon, but then he speaks spanish to his father.. i thought maybe his italian was bad so i tried really hard to understand him, but it was actually spanish

The actor's real parents are Colombian, so he's likely a native speaker of Spanish. The filmakers probably just went with it, instead of simply changing the character's ethnicity.

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Maybe his father is Spanish and his mother is Italian?

and the doctor said, 'you're er, seven years pregnant... or summin like that'

Simple, really: just because a person has a certain last name or ancestry, doesn't mean that he will speak that language. Most people in the U.S. who have a last name ending in "-ski" don't speak Polish, for example. Or Garcia's won't speak Spanish necessarily. It's the language of the family that is spoken, not the language of the last name. And by the way, yes, his Spanish is perfect, with a Colombian accent (like somebody previously said, his parents are Colombian).