The idea is to play whomsoever you like the sound of! Civ descriptions are coming shortly.
The five space agencies are designed to represent (roughly) the whole of Earth:
NASA: America
ESA: Europe, Russia
CSA: Britain, Canada, India, Australia, NZ, South Africa, other Commonwealth countries
ASEC: Japan, China, Thailand, Korea, Laos, Viet Nam, Cambodia
ISA: South America, Middle East, Indonesia, elsewhere.
The other civs either represent multinational corporations or independent coalitions of colonists drawn from everywhere.
Does this help?