add <link rel="alternate" hreflink=""> to pages

[stapelberg]

…so that search engines can deliver results in the best lanugage for their users. See also https://support.google.com/webmasters/answer/2620865?hl=en

[stapelberg]

Actually, the sitemaps contain references to the index pages of a package, which are always in english. Hence, we’ll need to add the tags to the manpages themselves, which is not trivial due to the way our templates are structured (header.tmpl cannot access manpage.tmpl-specific attributes).