Madagascar is the name of the island in English, French, and a number of other languages.
Malagasy can be used as a noun for the name of the people and the language of Madagascar. As an adjective, Malagasy describes anything from Madagascar (i.e. Malagasy vanilla)
Madagasikara is the name of the island in Malagasy (in Malagasy the letter "c" does not exist).
Malgache is the French word for Malagasy.
Madagascan is used by people who don't know better (i.e. haven't heard/don't understand the term "Malagasy"). >