Localize CLI output and documentation

### Area

Other / Not sure

### Problem or use case

The CLI currently uses English. This issue collects requests for other languages and volunteers to maintain translations. A contributor has offered Simplified Chinese translation and review in the comments.

### Proposed solution

Keep this demand-driven. Identify the first language, intended audience, and ongoing reviewer before selecting a library or converting output strings. Documentation translation can be a smaller first contribution.

If CLI localization proceeds, define the translatable human-facing output and locale selection with an English fallback. Keep commands, flags, JSON contracts, API enum values, environment variable names, and other machine-facing identifiers stable.

Verify fallback behavior and unchanged machine-readable output. No server API change is needed.

### Alternatives considered

Retain English-only output, or translate documentation first. There is no agreed list of languages or implementation schedule.

### Contribution

- [ ] I'd be willing to submit a PR (once it's `status:finalized`)

Start by identifying the CLI's human-facing output and the documentation areas proposed for translation; no files or tests are named in the issue. First establish the initial language, intended audience, and ongoing reviewer, then define the scope and acceptance checks for English fallback and unchanged machine-readable output.

Written by the indexing model from the issue text.