If you are in the U.S. and you are trying to determine the copyright status of a foreign created work, then the first presumption is that U.S. law applies to make that determination.
The Berne Convention, an international treaty, dictates a principle of National Treatment. National Treatment provides a form of reciprocity amongst member countries so that a foreign work will be protected in the U.S. in the same way and upon the same standard as would a work created within the U.S. Similarly, U.S. works would be given the same privilege of protection under the laws of any other member country of Berne.
Thus, the second presumption is that so long as you are carrying out your substantive scholarly work in the U.S. you should apply U.S. copyright law to make your copyright determinations on your own work and the work that you may be integrating into your own as an underlying work.
Additional issues may arise such as when you work with international partners, where work is created across borders and jointly amongst collaborators, particularly where collaboration or distribution takes place online.