`hreflang` support

[simonwheatley]

We should consider adding hreflang attributes, e.g. in the switcher links, as <link rel="alternate" href="example.com" hreflang="es-es" />, sitemaps.

There's also the OG alternate locale tags: <meta property="og:locale:alternate" content="fr_FR" />

Reference:

• https://support.google.com/webmasters/answer/189077?hl=en
• https://support.google.com/webmasters/answer/2620865?hl=en
• https://yoast.com/hreflang/
• http://moz.com/learn/seo/hreflang-tag