The issue was brought up again last year: https://community.filemaker.com/message/184200#184200.
Thai is part of a larger group of languages who's Unicode character sets cannot be properly put on display.
This hurts if a language-specific UI is required, outside of the supported range of languages, as the larger part of spoken languages is unsupported.
Is: Text fields and text objects and labels on solution layouts
Is not: Language support for FM's own built-in GUI.
I unfortunately don't speak Thai, but I had done some proof-of-concept testing when I saw earlier discussions of this problem, and I think it MAY be possible to use keystroke script triggers (e.g. OnLayoutObjectKeystroke) as a workaround, pending a real fix in FileMaker. it's obviously not an ideal solution, and my version probably doesn't cover all cases, but somebody who know both Thai and FileMaker can probably improve on this.
# Proof of concept for scripted handling of combined unicode character entry not accepted through normal keyboard entry
# Designed to address issues with entering Thai characters
Set Error Capture [ On ]
Allow User Abort [ Off ]
Set Variable [ $TriggerKeystroke ; Value: Get ( TriggerKeystroke ) ]
Set Variable [ $TriggerModifierKeys ; Value: Get ( TriggerModifierKeys ) ]
If [ $TriggerModifierKeys > 0 ]
# Add to global variable to use as accent
Set Variable [ $$Accent ; Value: Code ( $TriggerKeystroke ) ]
Set Variable [ $CharacterCode ; Value: Case ( $$Accent ≠ ""; $$Accent & "0" ) & Code ( $TriggerKeystroke ) ]
Set Variable [ $Character ; Value: Char ( $CharacterCode ) ]
Set Variable [ $$Accent ]
If [ $Character ≠ "" ]
Set Field [ Thai::ThaiText ; Thai::ThaiText & $Character ]
Exit Script [ Result: False ]