Final answer:
Sir Gawain and the Green Knight was originally written in Middle English, the vernacular language after the Norman Conquest, which depicted the culture of chivalry and courtly love.
Step-by-step explanation:
The poem Sir Gawain and the Green Knight was originally written in Middle English. Middle English developed after the Norman Conquest, becoming the language of literature and common communication. Old English, which existed prior to 1066 A.D., was the language of the old aristocracy, with Latin used for scholarly and religious texts.
During the later medieval period, noble and royal courts fostered a culture of chivalry and courtly love, which found expression in the vernacular. Works such as Sir Gawain and the Green Knight, as well as popular stories of King Arthur, were spread by troubadours and emerged from the tradition of oral storytelling.
Therefore, the language in which Sir Gawain and the Green Knight was originally written was Middle English, distinct from Old English and Latin—languages that were also in use during earlier and the same periods for different societal functions and texts.