We do have an existing Enhancement request for the Poland language internationalization and some of the other languages to be part of the future version.
Out of the box but customers would like to have them as part of product-feature.
However, it is possible to add a custom locale and localize the application using bundles. This procedure is described in JasperReports Server Admin Guide:
This involves a few steps:
You'll need to *.properties files and translate labels and messages in these files.
You may need to change date formats and ad-hoc masks from jasperserver_config.properties & adhoc_masks files as per your need.
This wiki article will give you an idea of how to edit *.properties files and add the translation for messages and labels:
Please, refer to the JasperReports Admin guide for the detailed description and create customized locales. Finally restart the jasperserver to reflect the changes in the JRS.
I hope this will help you!