Is that the dialogue that's supposed to start with "It is our custom to seal any such alliances with marriage." or is it a subsequent option?

It happens in both quotes, in subsequent option when I choose lady from his family to marry, and when I just tell him that I am in secret relationship with lady and ask for permission.

Try going through the dialog using English from the launcher and see if the problem persists, maybe the Polish translations are using the wrong string registers. Should be just a cosmetic bug.