Adds support for adding SEO-friendly hreflang html tags. From Wikipedia:
Hreflang specifies the language and optional geographic restrictions for a document. Hreflang is interpreted by search engines and can be used by webmasters to clarify the lingual and geographical targeting of a website.
After setting up which blogs supply which languages, and optionally which blog provides the x-default language, the hreflang links will be added automatically on all of the specified blogs.