Additional key naming and English translation errors about publii-ui-locales

[dziudek]

@rbenitezpagan - thank you very much for your feedback - all fixes will be included in the upcoming v.0.40 of Publii :)

[rbenitezpagan]

@dziudek Updating the keys in the other locales for support of 0.40.0 is something you guys would do or is that something I can help with?

I believe issues #1, #5, and #6 can be closed.

I submitted PR #9 which updates the README documentation to make it easier to identify available locales and its download link.

And finally, for future work, do you guys at Publii envision keeping the locales for the different versions like:

downloads
-| 0.39.0
  |-- en-gb.zip
  |-- XX.zip
-| 0.40.0
  |-- en-gb.zip
  |-- XX.zip

Or just the latest?

[rbenitezpagan]

@dziudek Probably errorDuringPreviewCreatinMsg can also be "creation" and not "creating" > errorDuringPreviewCreationMsg