Webmaster Trends Analyst, Pierre Far says, “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.”

The above sentiments by Far owe to the introduction of a new rel-alternate-hreflang annotation that the Webmaster can use to specify such homepages that is supported by both Google and Yandex. A practical example of the above can be obtained from the site example.com that has content that targets users around the world as follows:

The Webmaster can interpret the above pages by employing rel-alternate-hreflang using Sitemaps or using HTML link tags 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 newly introduced x-default hreflang attribute value reflects signals to Google’s algorithms that a particular page cannot target any specific language or locale and is the default page when no other page is better suited. This similar annotation is also applicable for home pages that dynamically alter their contents based on a user’s perceived geolocation or the Accept-Language headers.

Users who have any questions regarding this facility should visit Google’s internationalization Webmaster Help Forum.

Source: http://googlewebmastercentral.blogspot.com/2013/04/x-default-hreflang-for-international-pages.html