Taking into consideration that the English language evolution timeline is divided in Old English (410-1150), Middle English (1150-1450), and Modern English (1450-present), we know undoubtedly that The Canterbury Tales was written in Middle English. In relation to the dialect, because Chaucer wanted everyone to be able to read and enjoy this story, he didn't use the language for the court (French), nor the language for the church (Latin). Instead, used the vernacular (plain and popular language) English that was spoken by everyone.