It's really hard to scrape the English Wikiquote for quotes, as pages
contain different types of non-quote items: credits, external links,
citatons, quotes in their original language.
The French Wikiquote seems careful about always using the
Template:Citation for actual quotes, which clearly marks them, and can
be consumed by machines easily.
Have you considered doing the same for the English Wikiquote? (I
understand it's a big task to convert all pages, but the convention
could still be adopted by a gradual process)