Assuming books have language specified in the metadata (most do, if not they should), then make the French Virtual Library search expression
languages:"=French" etc. Might worth doing a search for
languages:false in All Books, that will list books with no language setting.
No point in polluting the subject tags with things for which there is a Dublin Core metadata element -- like Language, Publisher etc.
A books Language setting is also used elsewhere, you wouldn't want to spell check Анна Каренина against an English or French dictionary
.
BR