This document is UTF-8 encoding but Language in FP is "None"

So:

<meta http-equiv="Content-Language" content="en-us">
<meta http-equiv="Content-Type" content="text/html; charset=utf-8">
 

Hello world (switch to Thai keyboard; keep Times New Roman Font)

ฟหกด 

back again in English keyboard. Next, switch to Thai and then set Font to MS Arial Unicode:

ฟหกด

Note that they are not quite the same... maybe the raw Thai keyboard is Angsana?