Enable i18n

[pavish]

DRAFT PR - details to come

• [x] Use system default for login page
• [x] After logging in, use value stored for user
• [x] Ensure both en and js get loaded in parallel
• [x] Test a production setup
• [ ] Add docs for building po files in Wiki
• [ ] Build .po files in Dockerfile?
• [ ] Show language switcher dropdown in password change page and superuser creation page?

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• [ ] My pull request has a descriptive title (not a vague title like Update index.md).
• [ ] My pull request targets the develop branch of the repository
• [ ] My commit messages follow best practices.
• [ ] My code follows the established code style of the repository.
• [ ] I added 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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