As for the documents necessary for family benefits, it depends on the type of benefit one will apply for.
It is not the case that for each form of social assistance, the legislator indicated the need to submit a birth certificate, but this document is the only one that can prove that the child comes from a given parent and that is why this person (this parent) is entitled to child benefits. It will be difficult to prove this fact in any other way (i.e. without a birth certificate).
As for the obligation to translate the birth certificate and the legal basis in this regard, the fact that every document in the proceedings before the authority should be translated results, for example, from the fact that Polish is the official language on the territory of Poland.
If there is no birth certificate, go to the embassy to obtain one.
However, if the act has never been drawn up and there is no evidence confirming with certainty the date and place of the event (child’s birth), the most appropriate solution seems to be the determination of the content of the marital status record in non-litigious proceedings before the court.
Information on how a Ukrainian citizen can obtain a birth certificate in Poland: https://ukraina.interwencjaprawna.pl/the-woman-gave-birth-to-a-child-in-ukraine-she-only-has-a-document-from-the-hospital-she-did-not-have-time-to-obtain-a-birth-certificate-in-ukraine-they-are-already-in-poland-where-can-she-get-suc/