The french translation says “Ajouter Un Utilisateur À La Modération Drapeau” which is pretty bad and way too long.
But I can’t this string anywhere in the plugin *.php or *.js.

Can anyone help me identify what string this is to propose another translation for it?

[@jgasba Participant 8 hours, 1 minute ago]

This line comes from the the bbp styling pack plugin.
https://translate.wordpress.org/projects/wp-plugins/bbp-style-pack/stable/