Add getLanguageCode to src/Users/User.php shared methods

[johntiror]

Add to User.php:

class User implements UserInterface
{
    /** 
    * @return string
    */
    public function getLanguageCode() {
    }
}

The function MUST return the IETF language tag of the user's language if available, null otherwise.

Note:

• Telegram returns exactly the IETF language tag of the user's language
• Facebook returns the locale in the form en_US, in such case the function MUST return the first two chars

[polyskalov]

Viber supports user language too.

Quite a useful feature. @botman, there are some updates on this task?