react-i18next recently made some breaking changes, in particular in how they handle plurals.

• https://github.com/i18next/i18next/releases/tag/v21.0.0
• https://www.i18next.com/misc/migration-guide#v-20-x-x-to-v-21-0-0

We need a spike to understand:

• Does `compatibilityJSON` work for both the old and new forms?
• Can plugins add both formats in the message JSON file?
• Can we translate the old format to the new format on the backend? (Or add some other backend logic to allow plugins to work with old and new console versions.)