Update French locales (#4034)

* add missing locales for French translation

* accent "Media" in the front-end locales

* images => médias

* Change 'rapport' to 'signalement' in French locales to be more coherent

* fix typo

* remove duplicate EN locale

* translate missing locales

* update missing locale

* fix typo

* unify with "utilisateur⋅ice⋅s"

* address PR comments