Legal provisions for mandatory referendums at national level
Latvia
Answer
Yes
Source
Constitution of the Republic of Latvia (last amended in 2005); Articles 77 and 70
http://www.wipo.int/wipolex/en/text.jsp?file_id=190856
Law on National Referendums, Legislative Initiatives and European Citizens’ Initiative (last amended in 2012); Articles 1 and 11.