Implements hook_form_FORM_ID_alter() for node_type_form().
function translation_form_node_type_form_alter(&$form, &$form_state) {
// Hide form element if default language is a constant.
// TODO: When form #states allows OR values change this to use form #states.
$form['#attached']['library'][] = array(
'translation',
'drupal.translation',
);
// Add translation option to content type form.
$form['language']['node_type_language_translation_enabled'] = array(
'#type' => 'checkbox',
'#title' => t('Enable translation'),
'#default_value' => variable_get('node_type_language_translation_enabled_' . $form['#node_type']->type, FALSE),
'#element_validate' => array(
'translation_node_type_language_translation_enabled_validate',
),
'#prefix' => "<label class='form-item-node-type-language-translation-enabled'>" . t('Translation') . "</label>",
);
}