Who decides on the final form of the ballot text?

Paraguay'sConstitution of 1992 with Amendments through 2011

https://www.constituteproject.org/constitution/Paraguay_2011

Article 290: Of the Amendment [Enmienda]

After three years of this Constitutionhaving been promulgated, amendments at the initiative of one-fourth of thelegislators of any of the Chambers of the Congress, of the President of theRepublic, or of thirty thousand electors, through a signed petition[,] may berealized.

The full text of the amendment must beapproved by [an] absolute majority in the Chamber of origin. [Once] it isapproved, the equal treatment will be required at the reviewing Chamber. If ineither of the Chambers the majority required for its approval is not met, theamendment will be considered as rejected, [and] it may not be presented againwithin the time [t?rmino] of one year.

[Once] the amendment is approved by bothChambers of the Congress, the text will be remitted to the Superior Tribunal ofElectoral Justice to convoke a referendum, within a time period of one-hundredand eighty days. If the result of this [referendum] is affirmative, theamendment will be sanctioned and promulgated, incorporating itself into theconstitutional text.

If the amendment is derogatory, anotherone with the same subject may not be promoted before three years.

The procedure indicated for amendmentwill not be used, but [rather] the one for reform, for those provisionsaffecting the mode of election, the composition, the duration of [the] mandatesor the attributions of any of the powers of the State, or the provisions ofChapters I, II, III and IV of Title II, of Part I.a

ElectoralCode (Código Electoral Paraguayo – Ley No 834-96), art 259.

http://www.oas.org/es/sap/deco/moe/Paraguay2013/docs/CODIGO_ELECTORAL.pdf

Artículo259.- El refer?ndum es una forma de consulta popular que se celebrar? deacuerdo con las condiciones y procedimientos regulados en el presente código.

Laautorizaci?n para la convocatoria del cuerpo electoral por v?a del refer?ndumlegislativo, en cualquiera de sus modalidades, consultivo o vinculante escompetencia exclusiva del Congreso, y su tratamiento, en lo que corresponda, sehar? seg?n el procedimiento legislativo establecido en la Secci?n II 'Dela formaci?n y sanci?n de las leyes' de la Constitución y de losreglamentos de cada C?mara.

It is notclearly specified for mandatory referendums, but the Article 290 of theConstitution provides that the SuperiorTribunal of Electoral Justice convokes the referendum for constitutionalamendments.