In the HTML header, add link tag with hreflang attribute pointing at translated pages

This improvement consists in adding link tags with a hreflang attribute to the HTML header to let search engines index translated pages of the same content as such, for SEO purpose.

Some references:

• https://developer.mozilla.org/en-US/docs/Web/HTML/Element/link#attr-hreflang
• https://moz.com/learn/seo/hreflang-tag
• https://developers.google.com/search/docs/specialty/international/localized-versions