Polen, Woiwodschaft Großpolen, Pila (Schneidemühl), Boulevard am Fluss, 60 m, 26. August 2017 (Freilandfoto: Marek Idec)
Very likely it's not Pieris mannii. I found sources saying that it doesn't appear in Poland. Nevertheless when I compared my photos with others in books or in the internet it looks to me more like Pieris mannii. Could you figure out what butterfly it is, please? Thanks in advance.