When Should a Website Use Hreflang Instead of a Canonical Tag?

I’m working on a site with separate English and French pages that cover the same product. Each page is written for its own audience, but some of the content and page structure are necessarily similar.

I’m unsure how to handle the relationship between them in search: should they point to each other with hreflang tags, use canonical tags, or both? I’d also like to know what changes if one language version is only a partial translation.