I included simple patch to demonstrate what would require changing, by no
means it's supposed to be final. There may be a little more work to avoid
duplicities, but it does pretty much everything necessary.
I would really like to see this as a built-in in Django.
Until now, I have created a custom `template tag` in my project which
produces Google's `<link rel="alternate" hreflang="..." ...>` elements. I
have submitted my sitemap to Google and it has successfully verified.