Is this Heracleum sosnowskyi?
No, I don't think it looks like Heracleum sosnowskyi. To me, the plant in your pictures looks similar to Angelica sylvestris (which is common in Poland), but there are many species in Apiaceae with a similar apparence and I'm not familiar with all of them. Species in the Heracleum genus has leafs that are "lobed" and all leaflets aren't as separated as the plant in your pictures (overall, the leafs of Heracleum look quite different), and the stem is usually "swollen" where the leafs connect (i.e. the leaf stems are swollen). Plants of Heracleum sosnowskyi (and H. mantegazzianum) are also much larger, and they often have a fussy apparence because of the bristle hairs. 

Angelica sylvestris (picture from from wikimedia commons)