Update from OP. On EZE-AMS there was announcement and champagne for both J and Y at midnight European time, which was about 3 hours into the flight. Captain also went through the cabin congratulating everyone.

Then they ran out usual J champagne and after new year were serving some Prosecco with apologies, probably from Y .

Nice flight and lovely crew. Half of the passengers in J were Chinese and FA commented that it’s not unusual to see many Chinese passengers on New Year flights because they do not celebrate our new year, but they did not mind drinking champagne at count down .