Officially ( Technical Specifications For FileMaker Pro 13 | FileMaker ) Filemaker only supports: Worldwide English, French, Italian, German, Swedish, Japanese, Dutch, Spanish, Simplified Chinese, Korean and Brazilian Portuguese.
So my guess is that the support has not changed since version 11, where only partial thai support via unicode is supported.
Please submit a feature request for thai language support:
FYI WinSoft also distribute other international versions of FileMaker Pro:
but I can't see anything Thai specific unfortunately.