A major fire gutted one of Latin America’s most popular and respected museums on Monday afternoon and killed a firefighter on duty there. The cause of the blaze at the Museum of the Portuguese Language in São Paulo — or the Museu da Língua Portuguesa — an important landmark and cultural resource for the region, has not been determined. São Paulo’s governor, Geraldo Alckmin, told reporters that the firefighter, Ronaldo Pereira da Cruz, had died at a hospital. The museum is closed on Mondays, a fact that probably averted additional casualties. The museum, in a building in the historic city center that is more than 100 years old, is considered a trove of information on the history and evolution of the Portuguese language. Early signs suggest that the cultural loss may be minimized. “We probably have many things backed up,” Isa Ferraz, a curator at the museum, told the Brazilian television station Globo. Mr. Alckmin, who visited the scene on Monday, pledged his support, saying of the museum, “It will be rebuilt.”