Complex sentence: It was the language of the upper classes until the 12th century, after which it became the language of commoners in West Brittany ("Breizh Izel"), while the nobility, followed by the bourgeoisie, adopted French.
Split into simpler sentences: It was the language of the upper classes until the 12th century, after which it became the language of commoners in West Brittany ("Breizh Izel"). The nobility, followed by the bourgeoisie, adopted French.