Implements hook_field_available_languages_alter().
function field_test_field_available_languages_alter(&$languages, $context) {
if (variable_get('field_test_field_available_languages_alter', FALSE)) {
// Add an unavailable language.
$languages[] = 'xx';
// Remove an available language.
$index = array_search('en', $languages);
unset($languages[$index]);
}
}