Excerpt from an interview:
D: Oda-sensei, I have a question. If One Piece was set in the real world, which countries would the nine Straw Hats come from?
Oda: Well. I’ll just put what fits with the character’s image.
Luffy: Brazil
Zoro: Japan
Nami: Sweden
Usopp: Africa
Sanji: France
Chopper: Canada
Robin: Russia
Franky: USA
Brook: Austria