PR Checklist
### PR Structure
- [x] This PR has reasonably narrow scope (if not, break it down into smaller
PRs).
- [x] This PR avoids mixing refactoring changes with feature changes (split into
two PRs otherwise).
- [x] This PR's title starts is concise and descriptive.
### Thoroughness
- [x] This PR adds tests for the most critical parts of the new functionality or
fixes.
- [x] I've updated any docs, `.md` files, etc… affected by this change.
What
Add a stable plugin API that allows transforming translations before files are saved.
Add a plugin to embed fallback translations based on the default locale.
PR Checklist
### PR Structure - [x] This PR has reasonably narrow scope (if not, break it down into smaller PRs). - [x] This PR avoids mixing refactoring changes with feature changes (split into two PRs otherwise). - [x] This PR's title starts is concise and descriptive. ### Thoroughness - [x] This PR adds tests for the most critical parts of the new functionality or fixes. - [x] I've updated any docs, `.md` files, etc… affected by this change.What
Why
Close #658
Known limitations
N/A