The world of One Piece is big and characters are from all over it so if there were real they would probably be from different parts of the real world too.

In a manga SBS question, a fan asked what nationality the Straw Hats would be of if they were real. Oda gave the following as a reply:

Monkey D. Luffy: Brazilian
Roronoa Zoro: Japanese
Nami: Swedish
Usopp: African
Sanji: French
Tony Tony Chopper: Canadian
Nico Robin: Russian
Franky: American
Brook: Austrian
Jinbe: Indian

At a later SBS, Oda provided real-world nationalities for some characters that appear much later.

The Worst Generation:
Capone Bege: Italian
Jewelry Bonney: Australian
Basil Hawkins: Egyptian
Scratchmen Apoo: Chinese
Eustass Kid: Scottish
X Drake: Turkish
Urouge: Indian
Killer: Scottish
Trafalgar D. Water Law: German
Marshall D. Teach: Somali