The game treats PLC as a cultural union of Polish and Lithuanian cultures, so the Union Nation decision is the one you want.
The funny thing is that there's a picture for a specific event in the files, despite the code not using it. So if you add an decision named form_poland_lithuania to the game it gets a different picture to the generic one other custom decisions get.
The form_union_nation decision also lets you make;
Romania (alternate method)
Czechoslovakia
Poland-Lithuania
Yugoslavia (alternate method)
United Baltic Duchy
India
USA (if it somehow vanishes)
Scandinavia (for bear cultured nations)