Add Polish language

[przlada]

I want to merge this change because... it adds the Polish language. For sure translation is not my best skill but here is my attempt to add Polish language to the storefront. Any improvements are highly be appreciated.

Screenshots

Pull Request Checklist

1. All visible strings are translated with proper context.
2. All data-formatting is locale-aware (dates, numbers, and so on).
3. The changes are tested.
4. The code is documented (docstrings, project documentation).
5. Changes are mentioned in the changelog.

[przlada]

I moved this changes to #788