Sources used in polish collection which do not fall into CC0 category:
- “source”: “From the book.”, “username”: “narid” – seems to be actually book “Biały Kieł/White Fang” not CC0 AFAIK. First polish translation was done by Anna Trzeciakowska https://pl.wikipedia.org/wiki/Anna_Trzeciakowska and was published in 1926. Ms Anna seem to be still in good health.
- “source”: “https://www.gutenberg.org/files/6000/6000-h/6000-h.htm Project Gutenberg version of “Ironia Pozorow” by Maciej hr. Lubienski”, “username”: “hellbunnie” - license here is somewhat about freedom but probably not CC0 equivalent, author is still alive as mentioned by @Adrijaned
- “source”: “https://pl.wikipedia.org/wiki/Zachęta_Narodowa_Galeria_Sztuki”, “username”: “michalstepien” - wikipedia originating sentences