Nilfgaardian differs from Elder Speech in that much of its vocabulary seems to come from Dutch. In the Blood and Wine expansion of the Witcher 3, during the "Capture the Castle" quest, the Nazairi soldiers use a great number of Dutch profanities; this includes rotzak ("bastard"), schijtlijster ("coward"), stront ("shit") and piemellikkers ("cocksuckers"). They also use the word kusse, later explained by Regis to refer to a vagina. This is akin to Dutch kut and its older form kutte, which carry the same meaning.
Edit: Kusse is a crude way of saying vagina in Danish