Thanks Steven
I think that we could solve the problem of "the" word by using some workarounds in translation, on our side only: such as, if a first string in the window shows the number of items, then the confirmation string below in the string can just read "do you want to delete
le(s) fichier(s)", without number, taking care of both cardinalities in one sentence.
This would just be a short task for the translators, not for you (looking for and correcting the few translation entries with "the {0}" in English by removing "the" when needed or by using the above trick).
The only thing that could need some fixing your side, if confirmed, is the correct displaying of the "1" in the following strings (found in the translation file). Note that I haven't checked whether the problem occurs for each of them, as I'm not sure where they are all used
#Main.msg.Files1# 1 fichier
#Main.msg.SelFiles1# 1 fichier sélectionné
#Main.msg.DupFiles1# 1 doublon
#Main.msg.NewFiles1# 1 nouveau fichier
#Main.msg.Items1# 1 élément
#Main.msg.Albums1# 1 album
#Main.msg.Tracks1# 1 piste
#Main.msg.Plays1# 1 lecture
#Main.msg.Subs1# 1 inscription
#Main.msg.Artists1# 1 artiste
#Main.msg.Genres1# 1 genre
#Main.msg.Years1# 1 année
#Main.msg.MoreFiles1# 1 fichier supplémentaire
#Main.msg.ArtistsUpd1# 1 artiste mis à jour
#Main.msg.AlbumsCov1# 1 pochette d'album mise à jour