As the title says, what ethnic background do you think they came from?
Sanji: I'd say he's French because he speaks the language and the french are known for their cooking, womanizing, and nice clothes, and well, to me, it totally defines sanji :D
Mihawk: I'd say he's spanish, just because he looks like a spanish person to me
Zoro: I know this sounds weird, but I swear he's gotta be chinese, I don't know why, but he just gives me the feeling that he's from a chinese background.
Ussop: Somehow I get the feeling he's a mexican, just because in some pictures he reminds me of a mexican :P
Robin: Definatly Egyptian, she just strikes as one don't you think? :)
as for the others… I have no clue -_-