Seriously? You are going to walk from Paris to Santiago???? That's HIGHLY ambitious!
Yeah...you should probably learn a little Spanish, but you'll surely pick up plenty in the time it will take you.
Yes, there's a passport. I don't know what it GETS you, but you get it stamped at stops along the way, and it's a souvenir.
I'm pretty sure you can stay at places that have the shell on them, discounted or free. Someone else is better versed in that.
How would going on a Catholic pilgrimage possibly help someone, religiously, of another religion? I'm not sure I understand your logic.