Fatal error: Call to undefined function AppointFoxAdminController locale_get_display_language()

The following error is a known issue when running on PHP without Intl extension enabled:

Fatal error: Call to undefined function AppointFoxAdminController locale_get_display_language() in /web/htdocs/webtherapy/wp-content/plugins/appointfox/admin/CustomersController.php on line 129

FYI, we used PHP Internationalization extension (PHP Intl) for our translation module.
To fix this error, just enable Intl PHP extension at your web server.

Here some guidelines:
http://my1.php.net/manual/en/intl.installation.php