Upvote:3
Schengen visas are a 'use or lose' document...
On the expiry date of your visa all unspent leave is automatically depleted; so your 12 days will be forfeit when that happens.
You also forfeit any unspent leave if your visa is 'single entry' and you exit the zone.
Returning to your question: Yes, you can visit France and use your 12 unspent days if...
If both of those conditions are true, you simply need to be out of the zone before the visa expires. If either of those conditions are false, then your visa is no longer useful. Hopefully you included France in your itinerary when you applied, otherwise you will need to think up a plausible explanation when you clear immigration controls in France.