Why is "Thailand" spelled with an 'h'?

As we all know, "Thailand" is not pronounced with a /θ/ — so why is it spelled that way?

Is the 'h' vestigial? Does it represent some subtle phoneme in the Thai language, and if so, what is that difference?

A couple of observations:

• It is also spelled with an 'h' in French and German, so it's no accident of English. (To be fair, there is no 'h' sound in French, and the 'h' is sometimes silent in German, as in "Neanderthal".)
• The Thai language is a member of the Tai language family, which is not spelled with an 'h'.

Solution 1:

Like many languages outside Europe, Thai distinguishes between aspirated and unaspirated plosives (eg [tʰ] and [t]). These both occur in English, but they are not treated as distinct sounds, so it is usually hard for English speakers to hear and produce them reliably. The word "Thai" in Thai starts with an aspirated consonant.

To percieve the difference, consider the English words "tick" and "stick". In English, an initial 't' is usually aspirated, but the 't' in initial 'st' is not - you can verify this be holding a hand in front of your mouth when you say the words: you will feel the puff of breath after the 't' in 'tick' but not in 'stick'. In those contexts we readily pronounce the 't' differently, but don't notice we are doing so, and have difficulty making or hearing the difference outside that context.

Scholars believe that the many English words that are borrowed from Greek and contain 'ch', 'th' and 'ph' (eg 'chasm', 'theatre', 'physics') originally all had aspirated stops in Greek, though in Modern Greek as often in English they have changed to fricatives.

Solution 2:

The Thai language has multiple consonants that correspond to the letter T in the Roman alphabet. The difference between T and Th is a phonetic difference that is light, but does exist in some English words as well. Thai words written in the Roman alphabet with the letter T are pronounced without exhaling after the consonant, like the T in the English word what. Thai words with Th, however, are pronounced including the exhalation after the consonant, like the Th in Thomas.