It's not something you can apply for. It's part of signing the "contrat d'accueil et d'intégration" which you can only do when you receive your first carte de séjour. For more details, here is the website for the agency charged with the contract for foreigners (the English pages ): ANAEM - Agence Nationale d'Accueil des Etrangers et des Migrations

I have a friend here who is taking the 400 hours of language training. It's not the best way to learn French (IMHO) - all students who could not pass the language test are lumped together in one class, regardless of level. My friend does seem to be making progress, but there are much better language classes available for quite reasonable prices.
Cheers,
Bev