Declare alternate languages on translated pages

[jneidel]

This MR declare alternate languages explicitly, if a page is translated.

Why is this necessary? The sitemap already declares alternates:

            <xhtml:link
              rel="alternate"
              hreflang="{{ .Language.Lang }}"
              href="{{ .Permalink }}"
              />{{ end }}

That does work, but for ahrefs this is not enough:

(Feb 15 scan was after I deployed the fix on my site.)

After the fix:

Fix on npm run example:

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