Incomplete translations show up in search results

I've been hesitant to generate and publish HTML for half-done translations since there is a risk that search engines get confused by the duplicate content. A half-done translation will result in lots and lots of pages with English content. We emit a correct `` tag, so I guess it should be fine, but it's something to be aware of.

_Originally posted by @mgeisler in https://github.com/google/comprehensive-rust/pull/2189#discussion_r1670615250_