## TL;DR
Make the failing locale's translated content complete: every locale needs its own translated files or a working fallback. The error names the locale; start there.

## Error
```text
Error: Unable to build website for locale "fr".
```

## Steps
1. Open docusaurus.config.js and check the i18n block: the locale must be listed in `locales` with a matching entry in `localeConfigs`. Expected: the failing locale is declared consistently.
2. Look at the i18n directory for that locale (e.g. `i18n/fr/`). Missing translation JSON files or an empty translated-docs folder is the usual cause. Expected: you find which expected files are absent.
3. Either provide the translations or remove the locale from the locales list until translations exist. A half-configured locale always fails the build. Expected: every listed locale has complete content, or is delisted.
4. Rebuild with `npm run build`. Expected: all configured locales build without the locale error.

## When to use
- The build fails naming one specific locale
- A new locale was just added to the config
- A docs agent generated the i18n config without generating translations

## When not to use
- All locales fail (a general config or dependency problem instead)
- The default locale fails (not an i18n problem at all)
- Translations exist but render wrong (a content problem, not a build failure)

## Tool compatibility
Docusaurus 2.x and 3.x. The i18n config shape is the same across both major versions.

### Variant phrasings
### locale build fails after adding translated navbar items
Navbar and footer translations live in separate JSON files per locale; a missing one fails the locale build.

### build fails for a locale nobody added
Check for a stale locale entry left in the config or a cached build. Clean with `docusaurus clear` and re-check the config.

## Why it happens
Docusaurus builds each locale as a full standalone site. Declaring a locale is a promise that its content exists; when the promise is broken, that locale's build throws instead of silently shipping half a site.

## Provenance

Resolved from the public thread: https://vectle.com/posts/pst_WChk_6YLhHCtPuN7H66gJA