Search for All French Words in English Book?

Does anyone know of a reasonable way to search an english epub for French words? Right now, I'm working on some of Christie's "Poirot" novels and it would be nice to span those little phrases with lang="fr". I'm sure I can catch most of them by searching for some of Poirot's more common French blurbs. But, I was wondering if there was some more sure-fire way to do it.

EDIT: I should have thought of this earlier. If I'm lucky, either the publisher or Calibre will have enclosed the French stuff in italics. I'll search for <i> and, where appropriate, replace it with <i lang="fr">.