Spanish is the most prevalent language on the
Camino Frances, once you leave Orisson. Remember you are in Spain, and as a general thing the Spanish do not speak English as a second language to the same degree that most Europeans do these days. Never assume that someone will understand English.
As a global traveler for more than 35 years, I have learned that it always pays to learn at least some basic phrases in the lingua franca of the place you are going, regardless of where it is. It surprises the heck out of a Japanese pilgrim on the Camino to be greeted in Japanese by an American, or a Thai in Thai, a Flemand (Belgian) in Dutch, and so on. It certainly serves as an ice breaker. So, when in Spain...
In preparation for my first Camino this year, I bought the Berlitz tourist phrase book with CD course. I ripped the CD to my iTunes library and put it on my iPod. I listened to it repeatedly during my training walks. Bear in mind that many of the people you will encounter in shops (tiendas), pharmacies, albuergues and hostals, etc. will only speak Spanish. You will need at least a basic command of useful phrases to be understood.
Originally, I made a folded "cheat sheet" for the most needed phrases. Those that I could not remember, I used to point to at a check-in desk or sales counter. But carrying paper it tiresome, adds to weight, and the paper gets wet and tattered after a coupe of weeks in my pocket. Also, smart phone, verbal translators do not work as fast or as robustly as you might think. They do not work well for spur of the moment exchanges as well as old fashioned paper.
So, in preparation for my upcoming Camino, I am preparing a list of most needed phrases based on my experience and I will translate them in advance then copy the English and Spanish phrases to the Notes app on my iPod Touch. Each category will have a separate "note." Category examples include: buying stuff, getting an albuergue bed, getting a hotel or hostal room, finding a bus or train, asking for items at a pharmacy, etc. Then I will not need Wi-Fi or a cellular signal to have a conversation. More importantly, this approach does not increase my weight.
This said, after Spanish, the "second" language of the
Camino Frances seemed to be English, in some form. Just be flexible and polite...