If I remember correctly, only the templates of the language which you used at the installation will show up later in the system.
So when you used English at the installation of osTicket or your osTicket is an upgrade from an older version, only the English templates will be in the system.
So you need to either wait until the multi language support is FULLY implemented (currently the translations / multi language support is at phase 2 of 4) or install a new instance of osTicket with e.g. Polish as the primary language to get the polish email templates instead of the English templates. The email templates and other missing translations will be added in future versions.