The will and testament in Thailand must be written in English (or another foreign language) and in Thai. This is to be valid in the courts of Thailand.

The Will and testament in Thailand, list all the testator’s assets at the time of signing the document. In addition, it will indicate the person or persons who will be appointed as the executor of the will, the beneficiaries and how the assets should be distributed among the beneficiaries. Two witnesses will be appointed and must also sign to certify the Testator’s signature.
It is not necessary to authenticate this will.

Upon the death of a foreigner in Thailand, the person appointed as the legal executor of the Testament should bring the original document to the Thai Court. He will receive a court order for the execution of the terms of the wishes expressed in the document.
In the event that a foreigner makes a will overseas for properties in Thailand, the legal executor may receive a will court order.
But in this case only if it is drawn up in the legally acceptable format under Thai law.
However, it must be translated into Thai and authenticated by competent government bodies.
While including your Thailand assets in an overseas will is legally acceptable, additional problems can arise for the deceased’s family as all documentation must be translated into Thai and notarized by the government agency.
It is therefore also possible and suggested, to draw up a will in Thailand to administer one’s goods present in the Kingdom.

What happens in Thailand if a foreign citizen dies without leaving a will?

Under Thai law, six classes of legal heirs will inherit from the deceased person’s estate in the order described below:
Descendants
Parents
Full blood brothers and sisters
Half-blood brothers and sisters
Grandparents
Uncles and aunts
Source ThaiEmbassy.com 

LCS can draw up the necessary documentation and authenticate it as necessary. We also deal with prenuptial agreements and any type of contract.