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

Add to User.php:

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

The function MUST return the [IETF language tag](https://en.wikipedia.org/wiki/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