Fatal error: Class 'NumberFormatter' not found

All you need is:

apt-get install php7.0-intl

No need to change php.ini or do anything else. (Tested on PHP 7 on Ubuntu 16.04).

The most up-voted answer here has you uncommenting a .dll which will never solve anything unless you are on a Windows server!

You just need to enable this extension in php.ini by uncommenting this line:

extension=ext/php_intl.dll

For more details visit, Enable intl extension

This worked for me (Ubuntu 18.10, PHP 7.2.15)

sudo apt-get install php-intl
service apache2 restart