Miguel de Cervantes used Spanish for the original 1605 edition of Don Quixote.
Published in Madrid, the first part was titled El ingenioso hidalgo Don Quixote de la Mancha. Cervantes published a second part in 1615, completing the story of the self-styled knight and his squire, Sancho Panza. The novel is widely regarded as one of the foundational works of modern Western literature.
The spelling “Quixote” reflects historical Spanish orthography. Modern Spanish normally writes the title as Don Quijote de la Mancha, although English-language traditions often retain “Quixote.”
Don Quixote is sometimes described as the first modern novel because of its complex narration, self-awareness, and treatment of fiction and reality. Its original language was Castilian Spanish, not a separate language called “Castilian” in the sense of a different work language.