`hreflang` support

We should consider adding `hreflang` attributes, e.g. in [the switcher](https://github.com/cftp/babble/blob/post_meta/class-switcher-content.php) links, as ``, sitemaps.

There's also the OG alternate locale tags: ``

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

Start with class-switcher-content.php, linked from the issue, and review the referenced hreflang guidance. Identify the existing integration points for switcher links, sitemaps, and Open Graph locale metadata before defining the scope. Done should cover the agreed multilingual metadata surfaces and preserve valid locale values.

Written by the indexing model from the issue text.