Simplify the following text for a Grade 11 native English-speaking person.
Original: The last centuries before the Roman invasion saw an influx of mixed Germanic-Celtic speaking refugees from Gaul ( approximately modern day France and Belgium ) known as the Belgae , who were displaced as the Roman Empire expanded around 50 BC .
Simplified: The last centuries before the Roman invasion saw refugees from Gaul ( modern day France and Belgium ) known as the Belgae , who were displaced as the Roman Empire expanded .