Why did Middle Breton written works contain many French words?

The widespread inclusion of French words in Middle Breton written works, despite those terms being alien to everyday speech, is attributed to the fact that the Breton language was neither used nor formally studied in Roman Catholic seminaries within Brittany.1

Because these institutions served as centers for education and literary production, the lack of formal instruction in Breton meant that writers often relied on French, which was the language of the region's towns and the educated classes. Consequently, Middle Breton literature frequently lacked originality, as many works were translations or adaptations of French or Latin sources, leading to the heavy integration of French vocabulary into the written language.23