You should try to find a Facebook page where people post flat their are leaving and sharing, they are usually specific to each city so it depends where you are going. You could try https://www.immoscout24.ch/fr as well and https://www.immobilier.ch/fr/ Best luck ! It's not that easy unfortunately. Afaik some french people first take an Airbnb and start looking when they are in the city.

wgzimmer.ch might also work

I would suggest flatfox.ch because it's free to advertise unlike homegate and anibis.