Who decides on the final form of the ballot text?

Referendum Law

Art. 16 National Commission prescribes the forms in the preparation and implementation of a national referendum.
 

http://www.zakon.hr/z/359/Zakon-o-referendumu-i-drugim-oblicima-osobnog-sudjelovanja-u-obavljanju-dr%C5%BEavne-vlasti-i-lokalne-i-podru%C4%8Dne-(regionalne)-samouprave