score:9
The practical answers are Japan and South Korea, both ~6 hours away (plus connections) from Thailand, Korea being a smidgen closer. Both have very developed, mature ski facilities -- you may recall that Nagano, Japan hosted the 1998 Winter Olympics, and Pyeongchang, Korea will follow in 2018 -- that cater to all levels and needs. In Hokkaido, in particular, the skiing is sufficiently epic that there's a town called Niseko that caters pretty much exclusively to a hard core of international ski nuts. Start with Snow Japan and this random KNTO page.
But since taking the easy way out would be too boring for Travel.SE, here's some random alternatives:
Upvote:2
Almaty, Kazakhstan is 4 hours away from Bangkok, beautiful and high mountains around 4 skiing resorts, and on of them considered the largest “Chimbulak” in Central Asia! At an elevation of 2,200 metres (7,200 ft) above sea level. The resort area is about 25 kilometres (16 mi) south of Almaty city by Medeo road. It is popular for its mild climate, large quantity of sunny days and great amount of snow through the winter (from November till May).
Upvote:4
If you want to go south of the equator, there's Australia and New Zealand. Both of them speak English.
The most common complaints about Australian ski resorts are the high prices, long lift lines, and poor quality snow. On the other hand, I find snow gums very pretty.
I've heard that New Zealand tend to have club skifields that are smaller and not so much profit-oriented as the ski resorts in Australia.
Some Aussies go to NZ ski fields, but I haven't heard of New Zealanders going to Aussie ski fields. Draw your own conclusions.