Muslims are required to arrange their affairs such that their estates are distributed pursuant to Sharia law’s share system.
This is easily accomplished with assets located within a Sharia law country because estates are, by default, distributed according to Sharia law’s share system.
Ensuring assets located outside of a Sharia law country are distributed in accordance with Sharia law is not quite so straight forward, however. The reason is that many countries don’t recognize Sharia law and, absent advanced planning, will distribute estates in accordance with local law rather than Sharia law. Even if the country where the assets are located recognizes Sharia law estate distribution principles, they likely won’t recognize the concept of plural marriage, meaning assets passed to any wife, other than the first wife, would likely be subject to estate tax.