There does seem to be an issue with the current implementation for fetching the user's language string. We diagnosed the problem and have a 1 line code fix we'd love for you to implement and verify the correct behavior.
To do so, please edit:
/components/com_jfbconnect/models/loginregister.php
Around line 300, you should see:
$lang = $app->input->get(ApplicationHelper::getHash('language'), '', 'COOKIE');
$user['language'] = $lang;
Update that to:
$user['language'] = JHelperContent::getCurrentLanguage();
The previous method works if the Language Switcher is configured to use a cookie to store the language. The default option was changed to Session and we apparently didn't support that.
Please test and let us know if that gets you going. We will ensure this is included in the next release of JFBConnect as well.
Thanks,
Alex