Hi there, Nathalie!
Which content are referring to? Is it the content right below the first calendar:
N’hésitez pas à utiliser les liens IMPORTATION GOOGLE AGENDA & ICAL pour ajouter d’un clic les évènements à votre calendrier personnel !
You mentioned that the issue goes away when you switch to the Twenty Eleven theme, so I think you’re right and there is a conflict with your existing theme. You may want to reach out to the theme author to see if they can pinpoint the source of the text. But go ahead and confirm which content is causing the issue and I’d be happy to take a look.
Cheers!
Geoff