Legal provisions for mandatory referendums at national level

Vanuatu
Constitution 2006
http://www.wipo.int/wipolex/en/text.jsp?file_id=195746
86. Amendments requiring support of referendums A bill for an amendment of a provision of the Constitution regarding the status of Bislama, English and French, the electoral system, or the parliamentary system, passed by Parliament under Article 85, shall not come into effect unless it has been supported in a national referendum.