Like the inhabitants of Earth are called Earthlings, what are the inhabitants of Golarion called? Golarions? Golarian? Something completely different?
The answer is Golarian
This information, as far as I know, is not present in any of the book published until now but we can be pretty sure on Golarian since this term is given to us directly from a developer (Mark Moreland to be specific).
Golarian is the adjective form of the proper noun.
Source: Mark Moreland answer