I agree with Oda on Sanij,Zoro and Franky. Key words: eyebrow, food, manners, women, coca cola, elvis, beach boy, katana, dojo.
Nami's hair color suits better Germany than Sweden for me. Chopper could also be Norway because of the snow you know. Usopp is certainly southern Italy by looks. Brook is more like a Lichtenstein guy, maybe because of his kingdom past or something.
Now comes the hard part. For I would not put Luffy to Brazil and Robin to Russia. Luffy is more of a Japanese guy. And Robin more a histrorian or something, which could hint at a colonial GB archelogist in Africa.
The hardest part is Luffy for me, can't find a really suitable country. Maybe chinese or japanese or from the philipines or something. Mmm, maybe a belgian ehehe