Build the hreflang <link rel="alternate"> block for an article.
<link rel="alternate">
Slug of the form <date>-<type> (no extension)
<date>-<type>
Newline-joined <link> tags for every supported language plus an x-default fallback pointing at the English variant
<link>
x-default
Build the hreflang
<link rel="alternate">block for an article.