Added polish translation

[PiotrZPL]

As a native speaker of the Polish language and a contributor of this project, I added a translation for my language. It may not be perfect, but I don't see any problems with it so far.

[zzag]

It looks like this PR contains some unrelated commits (up to "Merge branch 'zzag:main' into main")

Can you rebase this PR against the current main branch? e.g. (not tested)

git remote add upstream git@github.com:zzag/plasma5-wallpapers-dynamic.git
git fetch upstream
git rebase -i upstream/main

and remove all unrelated commits so there are only

• Added polish translation
• Update pl.po

while on this, you could also squash those two commits, however I can do that as well

[PiotrZPL]

Is this what I was supposed to do?

[zzag]

Thanks!