It means on the French equivalent page there is a comment, which is not on the English page.
Also the other way round if there is something on the English page but not on the French they would see (en) on there alongside whatever the comment is, such as BU or anything.
I think BU is a grade like Brilliant Uncirculated. Yes, just seen this ...
https://fr.numista.com/numisdoc/estimer-l-etat-de-conservation-d-une-piece-71.html
and it is Brilliant Universel grade. There is also BE on there which is Belle Épreuve grade.
English
https://en.numista.com/catalogue/pieces81.html
French
https://fr.numista.com/catalogue/pieces81.html
Here is an example of a French page with (en) on it, saying Proof grade ...
https://fr.numista.com/catalogue/pieces10856.html Spain 1 Euro Cent KM# 1144