It was a bill in parliament in 2006. It didn't have a lot of support (10%) and opposition from Episcopate.
In 2016, there was a religious ceremony (in which MPs and President were participating), but there's a big uncertainty over exactly what the legality of it is (as opposed to merely spirituality). There was no parlamentary vote I could find proof of (although one article claimed a vote in April 2016)
After the ceremony, Bishop Andrzej Czaja said: "It is not a declaration of Christ the King, because he is king. It is not a declaration of Christ the King of Poland. His kingdom is not of this earth, and his dominion is over the whole universe," adding that we should recognise Christ's reign over the whole world and submit to his law and his will (source, translated from official Polish source).
Here are details of 2006 vote:
From December 22, 2006 post: (http://www.bosnewslife.com/2646-2646-polands-parliament-to-vote-on-naming-jesus-ki):
Conservative parliamentarians from the ruling Law and Justice party and League of Polish Families as well as the opposition Peasants Party drawn up this week's resolution naming Jesus Christ the King of their mainly Catholic nation.
**So far only 46 members in the 460-seat parliament back the proposal, well short of the 231 votes to pass, said Szymon Ruman, spokesperson for parliamentary speaker Marek Jurek. ...
The article adds credibility by citing specific Polish clergy leaders criticizing the move:
Some Polish church leaders criticized the move as unnecessary. "Christ doesn't need a parliamentary resolution to be the king of our hearts," said Monsignor Tadeusz Pieronek, a member of Poland's episcopate and rector at Krakow's Papal Academy of Theology.