Be prepared for most locals, including hosts, to have limited English along this route. The pilgrims I met with no French skills were still enjoying themselves, but knowing a few phrases went a long way.
Learn Some Basics
I suggest trying Duolingo, a free language-learning service for your browser or phone. They offer a ton of languages including French and you can begin your lessons at any level.
Reservation Email Template
Here is a template to make email reservations with hostels. English translations in brackets; substitutions below.
Je suis un pèlerin sur Le Chemin Compostelle, et je voudrais faire une reservation pour le 29 Avril (I’m a pilgrim walking El Camino, and I’d like to make a reservation for the 29th of April). Une personne pour une nuit, avec le petit dejeuner, si possible (One person for one night, with breakfast, if possible).
Mon numero (My telephone number):
Je vous remercie! (Thank you!)
-(Your name for the reservation)
Words to Substitute:
-“Pèlerin” is for male pilgrims, “Pèlerine” is female.
-Replace “le petit dejeuner” (breakfast) with “demi-pension” if you want dinner + breakfast.
-Replace “le petit dejeuner” (breakfast) with “supper seulement” (only dinner).
-“Avril” in my template means April. You can replace that word alone for a different month without changing the sentence.
If further information is required:
This template will cover what most hosts require for a reservation (food preference, number of people, dates), but if they reply looking for more specific information, Google Translate is surprisingly accurate. For one-word translations, Word Reference is a great resource.
Carrying a physical phrase book is just extra weight!