Is there a set of laws which govern international flights?
International Airline is bound to follow the law of country of registration. Hence your flight limits you to dringking law in Abu Dhabi which is 18

Note some airline, Turkish here, have some policy not to serve alcohol on certain flights. On my experience with Hong Kong airline, the flight in-bound to Hong Kong only serve juices where out-bound had Tsing Tao :)

A not a really related topic but something more to read on travel.SE : Is it legal to drink your own alcohol onboard a plane over international waters?