Most countries have the life plus 70 years rule, but best to check with a lawyer. AFAIK, US, Europe and Australia have the same method of calculation as Singapore.

How to know which rule applies? Check author's citizenship first. Singaporean authors will enjoy the protection given under Singapore copyright law. There are a lot of other factors to consider but that's quite a complicated process, and that's where the lawyers can help. (me no lawyer )

I don't think there's such thing as an international copyright... yet. It may happen in the far future. Unlikely, but not entirely impossible.