When to capitalize "Spanish":
* As an adjective: When referring to the language, culture, or people of Spain: "I'm learning the Spanish language." "She loves Spanish food." "He's of Spanish descent."
* As part of a proper noun: When part of a specific name: "The Spanish Inquisition." "The Spanish Civil War." "The Spanish Royal Family."
When to lowercase "spanish":
* As a general term: When referring to the language in a general sense: "She speaks spanish fluently." "He studied spanish in college." "The book was translated into spanish."
In short: Capitalize "Spanish" when it's used as an adjective referring to a specific entity (language, culture, people, or proper noun). Lowercase it when used as a general term referring to the language.