What is jetlag ?
It’s a word that doesn’t exist in French so they often use jetlag to describe the feeling. Jetlag is the feeling of tiredness when you’ve travelled by plane, and, because of the time difference, you get tired. It is not décalage horaire (time difference).
Jetlag is an entirely different concept, so you need a word for it.
Learning English gives you the opportunity to learn new words and concepts like this, that you might not have in your native language.