Complex sentence: After the Roman conquest of Gaul (finished in 51 BC), Romanization of the Celtic upper classes proceeded more rapidly than the less complete romanization of the lower classes, who may have spoken a Latin language mixed with Gallic.
Split into simpler sentences: After the Roman conquest of Gaul (finished in 51 BC), "romanization" of the Celtic upper classes proceeded more rapidly than the "romanization" of the lower classes. These lower classes may have spoken a Latin language mixed with Gallic.