I selected all books (687) noted as "English", right-clicked in the window, chose "Edit metadata in blulk":
then in "Search field", I clicked on "languages", in "Search for" I wrote "English" and in "Replace with", I wrote "French" !
Then I clicked on APPLY and OK !
But... when I went back to the list, the 687 would be "English" books were still there ! I tried exit the software and run it again, but... no change !
The "Edit Metadata Information" thing isn't user friendly IMO... maybe because I'm not of english mothertongue...

!?!