Have a look at the documentation chapter The Intl Component.
composer should automatically install symfony/intl: 1.0.*
if the intl extension isn't available. Please note that you're limited to the en
locale this way.
Check your phpinfo()
output if the php-intl
extension is available/enabled and if yes which ICU version it was compiled with.
If the extension isn't available/enabled and you have no way to install it ... you will need to use the fallback-version.