Because doing so is a terrible practice, and I'm saying this as someone who is not a native english speaker. The communication between the tower and the plane

Because doing so is a terrible practice, and I'm saying this as someone who is not a native english speaker. The communication between the tower and the plane is not just for their benefit. Everyone else is on the same frequency, listening in, and needs to be able to understand what is going on. Not everyone is going to be able to understand french, while everyone flying in controlled airspace is going to understand english.