Have you turned on the World-Ready Composer? It is accessible in all versions of InDesign CC,from many locations, including the Justification menu, and the flyout on the main toolbar:
But, to be honest, I have chronically had problems using Apple system fonts for SE Asian languages outside of software that was itself written by Apple. I'm at work right now in a PC shop, but when I get home I will pop open my MBP and take a look at this font to see if I can offer any other advice. I don't typically work in Thai but I can tell you that I wave my hands and shake my head dejectedly whenever someone mentions Thonburi.
To mark text as Thai (which should be unnecessary for correct vowel placement) there are a few ways to go about adding more languages to the Language dropdown.
Hi and thanks for your reply!
I've tried the world ready composer, but it doesn't work when the tracking or kerning is changed. The floating vowels still move. In fact when all I did was switch to optical tracking, everything moved up and over radically. Our interactive team has style sheets for all other languages set at +5, for readability, and so the Thai comes in all wonky. That being said, It's likely that I'll have to recommend that no tracking or kerning be used with this language. Our creatives won't like that at all, as they like to be able to control every facet. But, I'm told that the subtitles and headlines are rendered as images anyway, so we can fudge that.
It's weird with Illustrator, though, since there is no world ready composer outside of a plugin. No love for illustrator.
If you find out a way, i will be very grateful!
I thought that Sukhumvit was a Mac system font, but I was of course completely wrong. Turns out it's an iOS system font, so I couldn't do any testing at all. I assume that you purchased the font directly from Cadson Demak, and I'd take my questions directly to them. It's probably a font build issue.
If I were using a font where I knew the foundry wouldn't listen to my concerns, I might write a fistful of GREP styles that would remove tracking from the floating vowels. Or I might use Peter Kahrel's kerning table script. It's brilliant.
But the most important bit is this: I would also write my style guide so that tracking wasn't indiscriminately applied to all writing systems. That strikes me as a no-brainer. Perhaps your default Latin-script font needs some brathing room at body text sizes in the language(s) you read, and the easiest way to get said breathing room in your workflow is to use tracking instead of a different font or careful kerning. but once again I'd rather use the kerning table script than just track everything willy-nilly.
That being said, It's likely that I'll have to recommend that no tracking or kerning be used with this language. Our creatives won't like that at all, as they like to be able to control every facet.
If your creatives can't read Thai then they should absolutely have that control taken away from them. I'm a localization wonk, not a "creative" so perhaps I'm judging a bit harshly, but if they want to "control every facet" then they'd best understand exactly how Thai is supposed to look - an understanding best achieved by literacy.