# Task 15c — The language toggle

**Agent**: `content-i18n-migrator` · **Model**: **Codex** **Depends on**: 05 ·
**Parallel with**: 15a, 15b · **Blocks**: 15d **Worktree**:
`.agents/scripts/worktree.sh start 15c language-toggle`

## Why this is its own task

This is the part of the full guide that does not survive a mechanical port, and
it is the most likely reason task 15 failed twice.

Today `applyLanguage` walks a `translations.pt` map of **CSS selector →
Portuguese HTML** and overwrites `innerHTML` at each selector. It keeps an
`originals` Map to restore English. That design cannot survive the migration:
the selectors are page-structure coupling, and once the content is a collection
the Portuguese string already lives beside the English one.

## Deliver a decision, then an implementation

Write the approach down in `.agents/context/content-i18n.md` (or the rule file
it points at) **before** you build, because tasks 15d, 16, and 20 all depend on
it and there is currently no stated answer.

The realistic options:

1. **Server-render both locales, toggle visibility.** Simple, no hydration cost
   for text, doubles the HTML.
2. **Server-render the saved locale, islands re-render on toggle.** Smaller
   HTML; every island then needs both strings client-side anyway.
3. **Separate routes per locale.** Cleanest, but changes URLs, which touches
   publishing and every internal link — out of scope unless you argue for it and
   the report flags it as a plan change.

Pick one, say why, and note what it costs.

## Behaviour that must not regress

- `localStorage` key `ai-for-dummies-language`, wrapped in try/catch — previews
  disable storage and an unguarded read throws.
- `document.documentElement.lang` becomes `pt-BR` or `en`.
- `[data-lang]` buttons get `.active` and `aria-pressed`.
- Toggling language re-renders the active phase panel and every selector panel.
  Coordinate with 15a: the island must expose a way to do this.
- `client:idle` — page-wide, not urgent.

## Do not

- Create `src/pages/full-guide.astro`. That is 15d.
- Edit `src/content/config.ts` schemas belonging to other collections beyond
  what the toggle genuinely needs; if a schema is wrong, report it.
- Touch `verify.mjs`.
- Translate, rewrite, or "improve" any string. Both locales already exist in the
  collections. This is plumbing, not copywriting.

## Done when

- [ ] Approach written down where 15d, 16 and 20 will find it
- [ ] Toggle island built, `client:idle`, storage guarded
- [ ] Both locales verified on a real rendered page, not just in theory
- [ ] `pnpm run gate` green — the full gate
- [ ] 42 assertions intact
- [ ] Report states the contract 15d must satisfy
