Hi there, this string is from mass messaging feature, when you want to delete all messages that was creating by sending mass message.
Report – is the report for the sent messages, like how many was read, etc.
You should try mass messaging to understand 🙂
The translations should be approved by wp localization editors community, you can ask them to approve or just export .po file at the bottom of that page:
https://translate.wordpress.org/projects/wp-plugins/bp-better-messages/dev/fr/default
and upload to your languages folder.
