Certification for international flight and certification for export to certain markets are two different things. To sell to the US-based airlines, you need FAA certification; to sell EU airlines you need EASA certification. To fly into these countries, the aircraft should only need be certification by your country's aviation authority complying with ICAO guidelines. I post related information some time ago when C919 got its Type Certificate (TC).
So yes, C919 can fly internationally and land it other countries.