It seems netflix takes note of the subtitles you want in the profile settings... and only adds them after a while. 2 weeks ago I barely did see any thai subtitles. But after selecting in in the profiles i see them on many international and/or korean releases.
Update: it's now working for me. Not sure if this was a temporary glitch at the site's end, or if it was something at mine. Could possibly be to do with what VPN IP I was on - when it was having issues, I was using an England location, now I'm on a Japan one it's working. Perhaps that's unrelated though - I don't have time to go back and forth checking right now!