Render-link hook isn't generating locale specific paths when possible

[chalin]

While PR #4689 was a good step in the right direction of solving

• [i18n][design] Interpret absolute paths as locale specific #4688

It doesn't work as expected.

The simplest solution would be to always prefix a language code to the absolute paths of non-en section pages, that causes problems for our link checker, which requires the use of canonical links (in other words, it knows nothing and can't deal with redirects). I'll post more later.