Translation System Requirements

Thank you so much to @perty for this handy list.

1. Translations are editable by non-developers (and thus have a different deploy cycle than development work).
2. Obvious when translations are missing.
3. Obvious when a text is no longer in use anywhere.
4. Some locales have their own messages.
5. Some locales may not have all messages so no translation is needed for those messages.
6. There is a single source of truth, i.e. only one place for the translations.
7. Optionally, history of changes. Who and when changed a translation.
8. Fallback to a default language when translation is missing.
9. Contextual UI that shows the translator where the translation is used.
10. Translation suggestions based on prior work (within the project, but also using translation services)