L'habit ne fait pas le moine.
Weird that the hint translated the whole as "Clothes don't make the person" which is not an acceptable answer, but is pretty similar in meaning to the accepted answer "Don't judge a book by its cover." Both are still English idioms too.
You don't see every word or sentence on your first pass through a lesson. Later, when the bars weaken and you review it again, you usually see a new word or two. It makes reviewing lessons less tedious.
When I hovered over "moine" it translated it as monk, so I answered "The clothes don't make the monk" and it was accepted.
It now accepts both of these ('clothes don't ...' and 'don't judge ...').
The key is for people to report omissions. At some point (here in less than a week) DL will make the necessary adjustments.