Get a few dozen people to request it.
Why? I think the cost of creating a local sub-forum is very low, compared to the benefits of having a whole space for discussion in people's own country/language.
I would even go further and suggest you create in advance ready-to-use local forums for more languages. But at least, 2~3 people asking for it is sufficient, because it means they'll be able to have a public conversation about bitcoin it in their language.
Plus, Thailand is kind of the most symbolic country for Bitcoin – after all, we're using their currency, aren't we?
Oh, maybe I got confused somewhere.