Matt Cutts the spam head from Google described in his official video in order to help the Webmaster world, on how to tell Google multiple domains are related.
He says Google always determine to establish the best interpret relationships for the websites over the web, at the same time he also includes the facts of the quality content of the websites, like Google consistent desire is to deliver the right and informative content to the users, no matter form which country they belong.
You have the best way to assure the Google, how multiple domains are related, you can use somewhat like, hreflang=”language-region”, this can use inside of a particular webpage through the use of markup code that would be referring Google to discover other variations of the same domain.
Google does the best to interpret where the users land from and deliver them desired and informative page as they always aspect to appear before him on search. At the other end linking page with hreflang=”language-region” will help Google, in order to recognize and to deliver the right one page conveniently to the users, on search results.
The most important fact he includes, users need not worry about as Google seems this sort of linking as paid or unnatural, although he advises to make this kind of link organic as possibly you can. He also added that you need to be very careful, and you have to avoid stuffing the footer with several links to the similar domains, it seems a bit spammy.
Here you can see the video