[Feature] PDF downloads in French and Spanish

[fraserbennett]

Problem
PDF Downloads currently has only English available, even if the top-nav language is set to français or español

URLs:
https://docs.bloomlibrary.org/es/downloads/
https://docs.bloomlibrary.org/fr/downloads/

Expected behavior
If a localized page exists, show that page.
If no localized page exists, show the English-language page with a banner (in the requested language) saying that there is no localized page.