Most of the pilgrims on the route will be French speaking, (though there might be quite a few German speakers.) I found that there was a real sense of community on the Le Puy route, and other pilgrims were ready to help those who didn't speak French.
One woman from South Africa carried a mobile phone, worked out which gîte she wanted to reserve in by looking at the Miam Miam Dodo guide, and always found a French speaker willing to ring on her behalf. There are some French pilgrims who speak a bit of English, and also where there are tourist offices there is often someone who speaks some English. The main issue you might face without speaking much French is that it could be a little lonely, and you will probably love banging into fellow English speakers, as we are a fairly rare breed along the route.
All the best. It's a beautiful route, with lots of interesting historical towns and villages to walk through.
Margaret
PS Heather, there are some others who've walked the Le Puy route on the Forum, and I am sure they will jump in with help. I am going to be pretty much off-Forum from tomorrow as I am leaving for another walk in France.