Foreign certificates obtained outside Poland (except for the International Baccalaureate and the European Baccalaureate) must have a stamped official certificate (an 'apostille') or must be legalised. On this basis, foreign documents are considered valid in Poland.
For documents issued in countries being members of the Hague Convention of 5 October 1961, a document called apostille shall be obtained. You can find a list of countries and institutions that are responsible for issuing apostille HERE.
Apostille is a document drawn up in a language other than Polish and must therefore be translated into Polish by a sworn translator.
Legalisation, like apostille, confirms the authenticity of a document issued in a country other than Poland. It must be done if it is not possible to obtain apostille, because the country which issued the document has not adopted the Hague Convention referred to above. If the certificate has been issued in such a country, it means that it is necessary to apply to the institution which is responsible for legalising documents intended for the use abroad.