If you make a Will to cover the assets held in Thailand, then the laws of Thailand apply, if said Will was drawn up in Thailand. If you make a Will in a foreign country, drawn up in that foreign country, then the laws of that country apply. If you should make a Will in Thailand to cover ALL assets world wide, the laws of Thailand apply, but all Wills can be contested by any or all beneficiaries.
I have a Will to cover my few assets in Thailand, drawn up by a Thai lawyer. I have another Will to cover assets in the UK, drawn up by solicitors in the UK. Both state quite clearly that neither can make a claim against the other.