user_preferred_language() changed to method on the AccountInterface and changed to return the language code

The API function user_preferred_language() was changed to $user->getPreferredLangcode() and returns only the langcode, which then can be used to load the language if needed.

Given that most uses of the user preferred language reference other data keyed by language code, we found that loading the language is not too often in the requirements, so this makes it easier to retrieve the value.

The type argument has been removed and replaced with a separte method: and $user->getPreferredAdminLangcode()

Drupal 7:

drupal_mail('contact', 'user_mail', $to, user_preferred_language($values['recipient']), $values, $from);

Drupal 8:

drupal_mail('contact', 'user_mail', $to, $values['recipient']->getPreferredLangcode(), $values, $from);

Related to drupal_mail() and hook_mail() changes: http://drupal.org/node/1783398