What about foreign / overseas assets? Can a Will made in Singapore cover them?
Short answer: Yes, provided that it is scoped accordingly.
Long answer: It is not that straightforward. While your Will can cover the overseas assets (regardless movable or immovable), it depends on whether that country recognizes the Will made in Singapore to begin with.
In particular for immovable property, Singapore requires the will's execution to conform to the internal law in force in the territory where the property was situated.
They MAY also have other laws such as forced heirship or certain conditions to the assets itself which may end up causing your assets to be distributed different from how you wanted it to be. Take Singapore's HDB as an example, you can give it to your foreign friend, but if he/she is not eligible to hold it, instead of receiving the property he/she will receive the proceeds of it.
That being said, regardless of the country, the main purpose of the Will is always to best reflect the Testator's intentions which the Court(s) will generally try to follow. However, due to the different jurisdictions/legislations and possible Estate Taxes involved, what we always recommend is that your client consults the relevant lawyers / will writers in that respective country.
If this is your first Will, consider putting "Worldwide" as the scope first. While you may / may not have overseas assets currently, what this does is to have a "base plan" so such overseas assets are not left out. You can subsequently draft one through a lawyer / will writer in the respective country(s) and edit this one accordingly.