Add pl locale

[slavfox]

Added a Polish locale.

Pull Request Check List

• Added tests for changed code.
• Updated documentation for changed code. (N/A?)

Note the PR template seems to be a couple years out of date:

Note: If your Pull Request introduces a new feature or changes the current behavior, it should be based
on the develop branch. If it's a bug fix or only a documentation update, it should be based on the master branch.

[sdispater]

Thanks for your contribution!