The word "Madagascar" is spelled and pronounced identically in both English and French.
The French pronunciation uses a uvular R and more even syllable stress, but the spelling is unchanged.
The name's origins are debated:
France colonized Madagascar in 1896, making it a formal colony in 1897. The island gained independence in 1960. French remains an official language alongside Malagasy, and the country's name is unchanged in both languages—a rarity among former French colonies (compare: Côte d'Ivoire vs. Ivory Coast, Cameroun vs. Cameroon).
The indigenous name is Madagasikara (the country) and Malagasy (the people/language).