In addition to researching the national (domestic) law of Poland you will need to also research Poland's obligations under international law.
Article 38 of the Statute of the International Court of Justice sets out the sources of international law:
The Court, whose function is to decide in accordance with international law such disputes as are submitted to it, shall apply:
a. international conventions, whether general or particular, establishing rules expressly recognized by the contesting states;
b. international custom, as evidence of a general practice accepted as law;
c. the general principles of law recognized by civilized nations;
d. subject to the provisions of Article 59, judicial decisions and the teachings of the most highly qualified publicists of the various nations, as subsidiary means for the determination of rules of law.
The most important source of international law are international conventions, also known as treaties or international agreements. This guide will provide information on finding treaties where Poland is a party. Please see the International Legal Research Guide for more detailed information on how to conduct international legal research, including how to find other sources of international law.