Not sure why < tranquille > gets the red line for "peaceful"? It seems that tranquille/calme/paisible should all be accepted. Is this an issue with usage, or just one of the acceptable answers not covered?
https://www.larousse.fr/dictionnaires/anglais-francais/peaceful/600884
https://www.larousse.fr/dictionnaires/francais-anglais/tranquille/78084