To install an LP to Plesk:
For example,
# ./plesk_locale_8.1_build070206.14_nl-NL_install.sh
After you receive from the system the message about successful installing of the LP, the LP you created and installed can be used in Plesk, it appears automatically in the list of installed language packs (Server > Interface Management > Locales) and becomes available for choosing in all Interface Preferences forms, and on the Plesk login screen.