Add Polish translation

[mkljczk]

No description provided.

[samlich]

Thanks, and sorry I took so long.

Could I wait to merge this and ask that you translate a few more lines once I add them to the German file? I can probably add them this weekend (although I can't guarantee the quality of the German translations).

[samlich]

Finally got around to it. If you are able to translate the other lines as seen in the de.json file, that would be great, otherwise I can merge as is.

PeerTube's translation system is very basic, so one line (which is split into 4) may not translate well. It is printed as "Failed to parse timestamp for" {the full line where the error occurred} ", only " {the part of the timestamp that was valid} " of " {the full invalid timestamp} " is valid." An example would be 'Failed to parse timestamp for "3. [Chapter 3](#5m3ss)", only "#5m3s" of "#5m3ss" is valid.'