Node object language property changed to langcode

In Drupal 7 and before, the 'language' property on nodes ($node->language) signified the language the node is saved with. As part of a bigger unification effort to differentiate language codes from language objects in the API, this was renamed to 'langcode' ($node->langcode). Now if you see 'language', that should be a language object and not a language code.

Along with that change, two minor things were also changed for consistency:

1. The 'language' form element in the node edit form was renamed to 'langcode'.
2. The 'language' variable that is used for theming nodes ('#theme' => 'node') was renamed to 'langcode'. See the examples below for more information.

Drupal 7:

// Returns the language code of a node.
$node = node_load(1);
return $node->language;

// Unsets the language element on the node form.
function example_form_node_form_alter(&$form, &$form_state) {
  unset($form['language']);
}

// Returns the language code of a node as a renderable array.
$render_array = node_view($node);
return $render_array['#language'];

// Preprocesses nodes for theming. Sets a custom language code.
function example_preprocess_node(&$variables) {
  $variables['language'] = 'en-gb';
}

Drupal 8:

// Returns the language code of a node.
$node = node_load(1);
return $node->get('langcode')->value;

// Unsets the language element on the node form.
function example_form_node_form_alter(&$form, &$form_state) {
  unset($form['langcode']);
}

// Returns the language code of a node as a renderable array.
$render_array = node_view($node);
return $render_array['#langcode'];

// Preprocesses nodes for theming. Sets a custom language code.
function example_preprocess_node(&$variables) {
  $variables['langcode'] = 'en-gb';
}