Remote Langauge Mapping for Language/Country codes

I am working with a Drupal 7 site that offers multiple variants of English (en-us, en-au, etc.), and Brazillian Portuguese (pt-br). All source content is en-us.

One of our translation providers (Sovee) uses the language-code-plus-country-code convention of specifying source and target languages (such as "Translate en-us to pt-br"). Other translation providers (such as Google Translate) requires the two-letter versions (such as "Translate en to pt")

In /admin/config/regional/tmgmt_translator/manage/google I have specified the Google Remote languages mappings as:

   "remote_languages_mappings" : {
      "en-us" : "en",
      "pt-br" : "pt",
      "en-ca" : "en",
      "de" : "de",
      "es" : "es",
      "fr" : "fr",
      "ru" : "ru",
      "en-gb" : "en",
      "en-au" : "en",
      "el" : "el",
      "en" : "en",
      "nn" : "nn",
      "sv" : "sv",
      "tr" : "tr"
    }

However, the Google Translate plugin does not appear to be honoring those mappings.

As a test, I modified tmgmt_google.plugin.inc , and hard-coded the source language to be "en"

  protected function googleRequestTranslation(TMGMTJob $job, $q) {
    return $this->doRequest($job->getTranslator(), 'translate', array(
//      'source' => $job->source_language,
       'source' => 'en',
      'target' => $job->target_language,
      $this->qParamName => $q,
    ), array(
      'headers' => array(
        'Content-Type' => 'text/plain',
      ),
    ));
  }

and added 'en-us' and 'pt-br' to the $languages array:

  public function getSupportedTargetLanguages(TMGMTTranslator $translator, $source_language) {
    $languages = array();

    try {
      $request = $this->doRequest($translator, 'languages');
      foreach ($request['data']['languages'] as $language) {
        $languages[$language['language']] = $language['language'];
      }
    }
    catch (TMGMTGoogleException $e) {
      watchdog_exception('tmgmt', $e, 'Unable to retrieve a list of available languages.');
    }
    // My code below.
    watchdog('langs', 'languages=<pre>' . print_r($languages, TRUE) . '</pre>');
    $languages['en-us'] = 'en';
    $languages['pt-br'] = 'pt';
    return $languages;
  }

This "hack" does the trick, and allows the Google Translator to translate our source (en-us) documents to Portuguese (pt-br) -- but (other than quick tests) I'd really rather not modify a contrib module.
Is there a better way to get the translations to work for us?
(I also tried the patch at https://www.drupal.org/node/1999994 -- it did not address this issue.)