International law is law at national level
The question of determining which law applies is governed by international private law. However, contrary to what the name might imply, this is not an international but a national law.
Thus determining the international private law that is applicable is the first question to be asked. To this end, it must first be established which court is competent for resolving the dispute. The competent court then applies the international private law of its own jurisdiction in order to resolve any conflict of laws.
For example, if the competent court is English, it will apply English international private law, although this does not necessarily mean that it will consider English copyright law to be applicable. An English court may be competent to deal with the dispute (e.g. because of the defendant’s place of residence) but will apply Swiss law taking account of the elements submitted (e.g. the parties involved have Swiss nationality and the dispute is regarding a work created and supplied in Switzerland).