Once the email address typed, you have to click on the button called "Completez la demande d'inscription".
This is not correct, nor understandable in French.
Completez should be written with an "accent" on the first "e"
Anyway, the verb "Completez" is not the adequate one.
Pls. replace it by "Envoyez", which means "Send"
Another major improvement would be to send a e-mail in French (and not English) to the new potential subscriber
Because of this 2nd flaw in your script, less than 50% of French readers do confirm their subscription. It's a pity...
Below is a correct message in French that could be sent to the new potential subscriber :
"Cher M. ou Mme (name)
Vous souhaitez recevoir le flux de messages provenant du site (blog name).
Pour confirmer votre inscription, veuillez s'il vous plait, cliquer sur ce lien (confirmation link).
Nous vous souhaitons une bonne lecture de votre blog.
Cordialement
Feedburner"