Patron languages are stored on the preferences tab. You will need to double check what your library uses for the code or value of the Spanish language preference. The patron's language has to match up to the language on the book's catalog record in order for nightly to assign the book.
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Main Status - begins - A Advanced Search
Preference - Type - begins - patlang
Preference - Code - Equals/Begins - S [or SPA, double check you don't have a S/E for bilingual Spa/Eng books if you use the begins]
