I was getting this error in Laravel as I created a new app using the latest version (that is 6.2). Not sure why, the class would work locally but not remotely (on a deployment running in Ubuntu 18.04.3 on DigitalOcean to be precise).
I was using \ResourceBundle::getLocales('') to get a list of valid locales present on a website and then using in_array($needle, $array) to check whether a given locale is valid in PHP.
Here's how I fixed it.
composer require symfony/intl to install Symfony's Intl component.in_array calls with \Symfony\Component\Intl\Locales::exists($translation).