Google introduced a new rel-alternate-hreflang annotation for webmasters to specify international landing pages. It's called "x-default hreflang," and it signals to Google's algorithms that a page doesn't target a specific language or location.
"The homepages of multinational and multilingual websites are sometimes configured to point visitors to localized pages, either via redirects or by changing the content to reflect the user’s language," explains Google Webmaster Trends analyst Pierre Far. "Today we’ll introduce a new rel-alternate-hreflang annotation that the webmaster can use to specify such homepages that is supported by both Google and Yandex."
"The new x-default hreflang attribute value signals to our algorithms that this page doesn’t target any specific language or locale and is the default page when no other page is better suited," says Far. "For example, it would be the page our algorithms try to show French-speaking searchers worldwide or English-speaking searchers on google.ca."
If example.com/en-gb targets English-speaking users in the UK, example.com/en-us targets English-speaking users in the US, example.com/en-au targets English-speaking users in Australia, and exmaple.com/ tshows users a country selector and is the default page worldwide, then the annotation would something like this:
<link rel="alternate" href="http://example.com/en-gb" hreflang="en-gb" />
<link rel="alternate" href="http://example.com/en-us" hreflang="en-us" />
<link rel="alternate" href="http://example.com/en-au" hreflang="en-au" />
<link rel="alternate" href="http://example.com/" hreflang="x-default" />
The annotation can also be used for homepages that dynamically alter their contents based on users' geolocation or the Accept-Language headers.