This is a very important issue in Thailand, especially with foreigners from common law countries (e.g. US, England, and Australia.)
The law of trusts derives from the English common law (though it is now mostly codified into statute.) Thailand is a "[civil] code" country so trusts are not allowed.
If you are a foreigner from a common law country and you have assets in both Thailand and your country of origin, I strongly advise against executing a Thai will without first considering the effect it will have on your non-Thai assets and non-Thai estate planning documents. For example, if you already have a will in your country of origin (which disposes of your non-Thai assets) and then execute a Thai will, it could be said to have revoked your existing foreign will, depending on the circumstances.
Unfortunately, I have heard that there are some companies and organizations in Thailand that would put your entire estate-planning scheme into jeopardy by convincing you to execute a Thai will. They would do this not knowing that it could possibly revoke your existing will. Of course, you are still left with a little bit of a problem in that, as a general rule, foreign judgments are neither binding nor enforceable in a Thai court (e.g. from probate in a foreign country) although there is a possibility it could be used as evidence.
Again, the effect that a Thai will would have on you and your assets is an extremely fact-specific inquiry. If you have an estate of any size, I would recommend that you spend some time to consider your alternatives. There may be ways to organize your affairs which would not require any Thai estate-planning documents which has the added benefit of also saving you some money.
As usual, I've probably made things more complicated!
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Roger