Add internationalisation support

[wgresshoff]

❤️ Thank you for your contribution!

Description

Add internationalisation support. By adding a lang column and extendig the unique constraint the goal is achieved. This feature is available in the Administration interface as well.

Checklist

Ticks in all boxes and 🟢 on all GitHub actions status checks are required to merge:

• I'm aware of the code of conduct.
• I've created logical separate commits and followed the commit message format.
• I've added relevant test cases.
• I've added relevant documentation.
• I've marked translation strings.
• I've identified the copyright holder(s) and updated copyright headers for touched files (>15 lines contributions).
• I've NOT included third-party code (copy/pasted source code or new dependencies).
  • If you have added third-party code (copy/pasted or new dependencies), please reach out to an architect.

Frontend

• I've followed the CSS/JS and React guidelines.
• I've followed the web accessibility guidelines.
• I've followed the user interface guidelines.

Reminder

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1. You license your contribution under the same terms as the current repository’s license.
2. You agree that you have the right to license your contribution under the current repository’s license.

[wgresshoff]

Needs inveniosoftware/invenio-app-rdm#2967

[tmorrell]

Looks good to me. I'd like someone with alembic experience to give an additional approval though.

[wgresshoff]

This is based on an older version, there is a bug and a overburdened commit history...