Langkawi was not even the closest airport at the time when the aircraft turned back. Kota Bharu (WMKC) and Kuala Terengganu (WMKN) would have been some 200 km closer. Problem is, all of these 3 airports close at or before midnight local time, according to Malaysian AIP. The logical choice would be Penang, about the same distance as Langkawi, and open 24/7.