Because domestic carriers or foreign enterprises are always traveling foreigners and Middle English to be able to communicate with foreigners all over the world to understand is the same.
Because domestic or foreign inevitably has always foreigner walking. And English as a common language to communicate with foreigners all over the world to understand the same.
Because the flight in the country or abroad have foreigners walking. English as a lingua franca can communicate with the foreigners all over the world to understand too.