Language code cleanup

[TimidRobot]

Fixes

Fixes #23

• [Bug] Deeds & UX: Clarify gettext translation domain and Transifex resource slug #23

Description

This work is incomplete, but reflects the current state of the cc-licenses app:

• add warning to README
• Use deeds_ux instead of django as filename part / Transifex resource slug
• Update to use Django language codes (lowercase RFC5646 language tags)
• Remove redundant language codes/translations
• Remove 0% translated
• Update source PO files to contain keys in messages (to match behavior of Transifex)
• Normalize dates, where appropriate, to match Transifex.

Checklist

• My pull request has a descriptive title (not a vague title like Update index.md).
• My pull request targets the default branch of the repository (main or master).
• My commit messages follow best practices.
• My code follows the established code style of the repository.
• I added or updated tests for the changes I made (if applicable).
• I added or updated documentation (if applicable).
• I tried running the project locally and verified that there are no
  visible errors.

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