Thanks to Sotirios expert advise, the source of the problem was quickly located.
It is related to the unusual implementation of the language switcher in the landing page: the html tag for the link to the English version of the landing page (<p class="text-align-right"><a href="https://privacy.web.cern.ch"><span style="color:#ffffff;">English</span></a></p>, which is part of the content) overlapped with the menu bar that in the French version spreads over 2 lines.
The solution is to add some extra space before the html tag for the link to the English version: