Regions might require more than one ROM to work. All regions require the Black/White ROM.
Here's a list of currently available regions and requirements:
Kanto: Fire Red and Black/White ROMs
Hoenn: Fire Red, Emerald and Black/White ROMs
Unova: Black/White ROM
Sinnoh: Platinum and Black/White ROMs
Johto: Heart Gold/Soul Silver and Black/White