config_translation.php

/**
 * Submit handler to save config translation.
 */
function config_translation_form_submit(&$form, &$form_state) {
  $form_values = $form_state['values'];
  $language = $form_values['language'];
  $group = $form_values['group'];

  foreach ($group->getNames() as $id => $name) {
    $config = config('locale.config.' . $language->langcode . '.' . $name);
    $base_config = config_translation_get_base_config($name);

    // Pass base configuration to check and save only overrides.
    config_translation_set_config($base_config, $config, $form_values[$id]);
    $saved_config = $config->get();

    // If no overrides, delete locale file.
    if (empty($saved_config)) {
      $config->delete();
    } else {
      $config->save();
    }
  }
  drupal_set_message(t('Updated @language configuration translations successfully.', array('@language' => $language->name)));
  $form_state['redirect'] = $group->getBasePath() . '/translate';
}

function config_translation_get_base_config($name) {
  config_context_enter('config.context.free');
  $base_config = config($name)->get();
  config_context_leave();
  return $base_config;
}