Will your Last Will and Testament that you've written to comply with U.S. laws work in another country where you own assets? Like many aspects of estate planning, the answer to this question is, "it depends."
Will Your U.S. Will Be Valid Under Foreign Laws?
If your will has been created and signed with the proper formalities as required by the laws of your U.S. state of residence and is considered a valid will there, nonetheless, it may or may not be valid in another country.
For example, if the laws of your state require a will to be signed only in front of two witnesses but the laws of the other country where you own assets require a will to be signed in front of three witnesses, then your valid U.S. will will not be valid in the other country.
Aside from this, while some U.S. states recognize "nuncupative wills" and/or "holographic wills," other countries do not. (A nuncupative will is a verbal will that is spoken in front of two or more witnesses, while a holographic will is one that is written entirely in the Testator's own handwriting and signed and dated by the Testator.) So if you make a nuncupative will or holographic will that is valid in your state of residence but the other country where you own assets does not recognize this type of will, then your U.S. will will not be valid in the other country.
Other Problems to Consider
Certain parts of your U.S. will may not be valid due to the unique laws of the other country where you own assets.
For example, in all U.S. states you are free to leave your property to whomever you want, except that if you are married then all states except for Georgia will protect some part of your estate for the benefit of your surviving spouse. On the other hand, many foreign countries follow the rules of "forced heirship," meaning that all or a portion of a decedent's property must go to a blood relative instead of to a spouse.
Another example of how a U.S. will can create problems in a foreign country is when the U.S. will establishes testamentary trusts for some or all of the beneficiaries named in the will. While trusts of this type are quite common in the U.S., they are not in many foreign countries due mainly to adverse tax consequences. This could result in substantial taxes being paid to the foreign country which will significantly decrease the amount of foreign assets that the trust beneficiaries will be able to receive.
What You Should Do
If you are a U.S. resident and you own assets located in a foreign country, then sit down with an attorney who is familiar with the probate, trust, estate and tax laws of the foreign country. This will enable you to determine if your U.S. will will pass muster in the foreign country or if you will need to create a separate will to deal with your foreign assets.