I plan on travelling by foot on the Camino Francés alone.
I will have access to the internet/google translate via my smartphone. I am also a native french speaker (from Quebec).
How much Spanish do I need to know for travelling within Spain?
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It's hard to answer "how much" but I'll try to provide some guidelines and background information. I haven't walked the Camino, though, so maybe some first hand information would be better.
Along the path, you will go through a majority of small villages. In these villages, do not expect local people to know any other language than Spanish. Maybe people in charge of the Albergues may be able to speak some English or French.
That said, in Spain the second language (or third, depending on the zone) taught in schools is English, followed by French. So people in their 20's or 30's should be able to help you out. Older people might have been taught some French, just Spanish, or didn't go to school (post-war era was really rough in Spain).
Also, in bigger cities along the path (Pamplona/Iruña, Logroño, Burgos, León or Santiago) it should be easier to communicate in English.
I think it's a good idea to write a cheat sheet with basic things (yes/no, how much is it?, and so on) and carry it with you, just in case you run out of batteries.