Final answer:
Chaucer wrote 'The Canterbury Tales' in Middle English, the language of England from 1150 to 1500, moving away from the aristocratic French and scholarly Latin of the time.
Step-by-step explanation:
Geoffrey Chaucer chose to write The Canterbury Tales in Middle English, which was the vernacular language of England from about 1150 to 1500. At the time, French was the language of the new aristocracy following the Norman Conquest, and Latin was the language of the Church and educated elites across Europe. However, Chaucer's use of Middle English grew out of a tradition where culture, especially poetic and artistic forms like troubadour poetry, was expressed in vernacular languages. The Canterbury Tales represents a pivotal move towards the use of the English language in literature, marking a departure from the previously dominant French and Latin.