The general rule for alcohol and almost everything else that isn't related to things like ATC and ICAO requirements and customs and immigration formalities are the rules of the country of registration apply until you cross into the aircraft, not onboard. It comes from the days of old when ships where the method of choice (only choice for a lot of routes) and it has never been expected that as you would change things on board as long as it doesn't leave the ship. Of course international law still applies especially regarding equipment, crew, pollution, and the various smuggling types of things. Almost all of the treaties (at least the modern ones) have clauses that allow for some local rules to be enforced and also allow the local regulators to enforce the treaty provisions when you're in their jurisdiction.