Hi,
Best way would be to translate via the translate.wordpress.com website. Because that is the future-proof solution and more people can contribute to and have benefit from the translation.
But the existing translation is quite old, because there is no active translator. So it will need some work before a language pack will be released via WordPress (at least 90% needs to be translated).
Second option:
- open the Polish .po file in the languages folder in Poedit
- Update the catalog (choose from the POT file).
- Add the translations that you need and save.
I can send you an updated catalog (steps 1 and 2), if you like. I won’t be able to help with the translations ??
Disadvantages of second option: only you will have benefit from the Polish translation. And you need to make a backup of it because when you update the plugin, the file will be overwritten.