Oh, I see the problem. This is the same problem I described in Always use %{count} variable when translating pluralized strings.
It should be one {# réponse} instead of one {1 réponse}. So, replace the “1” inside the one case with “#” to make it work. The admin_js.admin.user.delete_all_posts_confirm_MF string is affected too.
You can fix the French translations at translate.discourse.org.
We should probably update the English strings to always use the # symbol in order to make it easier for translators.