RESOLUTION
Since Plesk 7.5 language packs are managed through Plesk CP on the Server->Interface Preferences page.
In Plesk versions below 7.5 unnecessary language packs can be removed with help of the /usr/local/psa/bin/lp_manage.sh utility.
Since Plesk 7.5 language packs are managed through Plesk CP on the Server->Interface Preferences page.