I was having the same problem as you finding videos with Thai subtitles. Then I read a post on this forum about some additional search buttons that the “Reactor” plugin adds on the youtube search bar. These buttons allow you to search for videos on a given topic that have captions in the language that you are interested in. It turns out there are quite a bit out there.
You can check out this thread which explains how to use the search feature: Youtube Extension - Now Online!