Implements hook_form_FORM_ID_alter() for language_admin_edit_form().
function locale_form_language_admin_edit_form_alter(&$form, &$form_state) {
if ($form['langcode']['#type'] == 'value' && $form['langcode']['#value'] == 'en') {
$form['locale_translate_english'] = array(
'#title' => t('Enable interface translation to English'),
'#type' => 'checkbox',
'#default_value' => locale_translate_english(),
);
$form['#submit'][] = 'locale_form_language_admin_edit_form_alter_submit';
}
}