In case it's useful to anyone else, here are the settings I used to integrate Google Translate with my site:
$wgWikiShareServices['GoogleTranslate']['service'] = 'Google Translate';
$wgWikiShareServices['GoogleTranslate']['url'] = 'https://YOUR-GOOGLE-TRANSLATE-SUBDOMAIN.translate.goog/index.php?title=%title%&_x_tr_sl=en&_x_tr_tl=es&_x_tr_hl=en-US&_x_tr_pto=nui';
$wgWikiShareServices['GoogleTranslate']['icon'] = 'https://upload.wikimedia.org/wikipedia/commons/d/db/Google_Translate_Icon.png';
You'll need to replace YOUR-GOOGLE-TRANSLATE-SUBDOMAIN with the correct value for your site which should be your site's main URL with any dots (.) replaced with hyphens (-). You can also go to Google Translate (https://translate.google.com/) and post a URL in the translation box, then click the link that is provided in the selected language. That URL will have the subdomain string you'll need.