I have tried to translate to Polish some parts of the player - weather-config.txt and radio folder names under radio-stations folder. If you want to see the rest labels in Polish you need to translate/localize the file labels.properties. You can get the files which I prepared here:
https://github.com/project-owner/Peppy. ... Polish.zip
Just unzip it in the 'languages' folder. I grabbed radio playlists from the English-USA folder and left just one station in each genre just as an example.
If you will face any issue please let me know either here or via e-mail peppy.playerATgmail.com
Here are the screenshots with my partial translations: