Added PL Language Pack

[VeeoN2]

Added Polish Language Pack

[ChocoMeow]

Thank you for your pull request adding Polish language support to our project! However, I noticed some issues in your language pack:

Key playlistNotInvalidUrl is not found in PL.json.
Key setStageAnnounceTemplate is not found in PL.json.
Key setPrefix is missing parameter 0.
Key pingfield1 is missing parameter 2.
Key pingfield2 is missing parameter 1.
Key playlistNotInvalidUrl cannot exist in PL.json.
Key trackLoad_pos is missing parameter 2.
Key SetStageAnnounceTemplate cannot exist in PL.json.

Please take a look at these issues so we can improve the language support. Thank you!

[VeeoN2]

I've fixed issues in PL.json file, however keys: pingfield1 and pingfield2 seem to be correct (identical to those in other language files) so i left them as they were.

[ChocoMeow]

it looks great. thank you so much <3