APS add-ons (e.g. WordPress localizations) are available on Applications tab, but cannot be installed on Parallels Plesk Panel 10.3 or newer with the following error:
Add-ons for APS packaged web apps are not supported.
In Parallels Plesk Panel 10.3, APS backend was rewritten and support for APS add-on packages was removed.
In one of the future versions the Application Vault interface in Parallels Plesk Panel will be reworked to hide the add-on packages. Add-ons for APS applications are not supported any more.