This is what I do, which works for me.
In the dashboard I go to System & Settings > Basics > Languages
There I click on the top right button that says "Install/Update Languages". It takes me to a different page where I can install different languages. I installed French.

Then I go back to the previous page where I now have the choice of language. I select French as the default language (I didn't check the option to have the choice on login) and I save and the change is instant, my interface is in French.

Thank you for your answer, I will try,
or is the tab to activate the maintenance page thank you ?
You understand French because I use google translate.

Oui je parle le Français et je n'ai pas vraiment compris votre dernière question. Que vouliez-vous dire au sujet du mode maintenance?