Use structurized JSON files instead of PO

[dominik-zeglen]

I want to merge this change because it resolves #383
Also fixes all messages with plurals.

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. Translated strings are extracted to .pot file.
4. Number of API calls is optimized.
5. The changes are tested.
6. Type definitions are up to date.
7. Changes are mentioned in the changelog.

[codecov]

Codecov Report

Merging #403 into master will decrease coverage by 0.04%.
The diff coverage is 37.5%.

@@            Coverage Diff             @@
##           master     #403      +/-   ##
==========================================
- Coverage   72.45%   72.41%   -0.05%     
==========================================
  Files         639      639              
  Lines        7120     7127       +7     
  Branches     1184     1185       +1     
==========================================
+ Hits         5159     5161       +2     
- Misses       1720     1725       +5     
  Partials      241      241

Impacted Files	Coverage Δ
...buteBulkDeleteDialog/AttributeBulkDeleteDialog.tsx	100% <ø> (ø)	⬆️
...onents/HomeProductListCard/HomeProductListCard.tsx	94.44% <ø> (ø)	⬆️
...ts/OrderBulkCancelDialog/OrderBulkCancelDialog.tsx	71.42% <ø> (ø)	⬆️
...ts/HomeNotificationTable/HomeNotificationTable.tsx	76.92% <ø> (ø)	⬆️
...nDialog/ProductTypeBulkAttributeUnassignDialog.tsx	100% <100%> (ø)	⬆️
src/components/Locale/Locale.tsx	56% <28.57%> (-10.67%)	⬇️

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