Who decides on the final form of the ballot text?

Political Constitution of Guatemala (Constitución Política de la República de Guatemala) (1985 with amendments in 1993)http://www.constitutionnet.org/files/Guatemala%20Constitution.pdf

Article 173.Referendum Procedure.Political decisions of special significance will have to be submitted to the referendumprocedure involving all citizens.The referendum will be called by the Supreme Electoral Tribunal on the initiative of thePresident of the Republic or the Congress of the Republic, which will determine precisely thequestion or questions to be submitted to the citizens.The Constitutional Electoral Law shall regulate what is relative to this institution.

No explicit provisions in this regard, but it seems that the text submitted by the entity that initiates the process is not subject to change.