This PHP extension is already installed by default for all Plesk PHP versions:
# cd /opt/
# find . -name intl.so
./plesk/php/5.4/lib64/php/modules/intl.so
./plesk/php/5.5/lib64/php/modules/intl.so
./plesk/php/7.1/lib64/php/modules/intl.so
./plesk/php/7.0/lib64/php/modules/intl.so
./plesk/php/5.6/lib64/php/modules/intl.so
./plesk/php/5.3/lib64/php/modules/intl.so
./plesk/php/5.2/lib64/php/modules/intl.so
# grep -R intl.so *
plesk/php/5.4/etc/php.d/intl.ini:extension=intl.so
plesk/php/5.5/etc/php.d/intl.ini:extension=intl.so
plesk/php/7.1/etc/php.d/intl.ini:extension=intl.so
plesk/php/7.0/etc/php.d/intl.ini:extension=intl.so
plesk/php/5.6/etc/php.d/intl.ini:extension=intl.so
plesk/php/5.3/etc/php.d/intl.ini:extension=intl.so
plesk/php/5.2/etc/php.d/intl.ini:extension=intl.so
For PHP5.6, for example, this module is loaded:
# /opt/plesk/php/5.6/bin/php -m | grep intl
intl