# Locale and Language Switcher

## Objective

Let people change language without losing their place or seeing half a translation.

A locale selector wired into routing and persistence, with sane fallbacks when a translation is missing.

## Before You Begin

This feature is being added to an application that already exists and already
works. Do not scaffold a new project, and do not assume a blank slate.

Inspect the codebase first and establish:

- The existing application structure and where code of this kind already lives.
- The framework and version in use.
- The existing design system — colours, spacing, typography, and component conventions.
- Existing UI components you can reuse instead of writing new ones.
- The existing database structure, if this feature needs to persist anything.
- The existing authentication and authorization system, if this feature is user-scoped.
- Dependencies already installed, so you don't add a library that duplicates one.
- The existing test setup and conventions.

Only start writing code once you understand the above. If the application
already implements part of this feature, extend it rather than replacing it.

## Implementation Instructions

1. Decide how locale is carried before building the switcher: a URL path segment, a subdomain, or a stored preference. The URL is the only one that survives sharing and indexing.
2. Resolve the active locale in a fixed order — explicit URL, saved user preference, Accept-Language header, then default — and apply the same order on the server and the client.
3. Switching must land the user on the equivalent page in the new locale, not on the home page.
4. Fall back per key to the default locale when a translation is missing, and log the miss so it can be filled. Never render a raw key like users.profile.title to a person.
5. Set the document language and direction attributes from the active locale, and support right-to-left layout where a supported locale requires it.
6. Do NOT infer the language from IP geolocation. Country is not language, and overriding an explicit choice with a guess is worse than showing the default.

## UI and UX Requirements

Match the application's existing design system exactly. Reuse its components,
spacing, and typography. This feature should look like it was always there.

## Responsive Requirements

Works on mobile, tablet, and desktop. Touch targets are large enough to hit on a
phone, and nothing overflows horizontally at 320px.

## Accessibility Requirements

- Fully keyboard navigable.
- Correct semantic elements and ARIA roles.
- Visible focus states.
- Meets WCAG AA contrast.
- Dynamic changes are announced to screen readers.
- Respects prefers-reduced-motion.

## Edge Cases

- The server-rendered locale must match what the client resolves, or hydration mismatches will flash the wrong language.
- A saved preference must not silently override an explicit locale in the URL — the URL wins, and it should update the saved preference.
- An entire missing locale should degrade to the default cleanly rather than producing a page of empty strings.
- Pluralization rules differ by language; a two-form English rule breaks in locales with more forms.
- User-generated content is not translated. Do not run it through the translation layer or label it with the interface locale.
- Right-to-left flips layout, icons with direction, and scroll position — mirroring the text alone leaves a broken page.
- Emails, PDFs, and other out-of-band output need the recipient's stored locale, not the locale of whoever triggered them.

## Testing

Exercise the feature end to end in the running application. Cover every edge case
above, then run the existing test suite and confirm nothing regressed.

## Acceptance Criteria

- [ ] Locale resolution follows one documented order on both server and client.
- [ ] Switching language keeps the user on the equivalent route.
- [ ] The choice persists across sessions but never overrides an explicit URL locale.
- [ ] Missing keys fall back to the default locale and are logged; no raw keys are ever rendered.
- [ ] There is no hydration mismatch or flash of the wrong language.
- [ ] Document language and direction attributes reflect the active locale, and RTL layouts are correct.
- [ ] Emails and generated documents use the recipient's locale.
- [ ] The feature matches the existing design system.
- [ ] No existing functionality is broken.

## Adaptation Rules

- Match the existing design system. Do not introduce a new colour palette,
  spacing scale, or component library.
- Reuse existing components and utilities wherever they fit.
- Follow the naming, file layout, and code style already present.
- Do not upgrade, replace, or remove existing dependencies to make this
  feature fit. Adapt the feature to the app, not the app to the feature.
- Do not break existing functionality. If a change is genuinely required in
  existing code, make the smallest one that works and say so.
- If something in these instructions conflicts with how the application is
  built, follow the application and explain the deviation.

## Final Verification

Before you report the work as done:

1. Re-read the acceptance criteria above and check each one against what you
   actually built.
2. Run the application and exercise the feature end to end.
3. Run the existing test suite and confirm you have broken nothing.
4. Check the feature on mobile, tablet, and desktop widths.
5. Check keyboard navigation and focus handling.
6. Summarize what changed: files added, files modified, and anything you
   deliberately did differently because of how this application is built.

If any acceptance criterion is unmet, fix it before reporting completion.