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:

Server-render both locales, toggle visibility. Simple, no hydration cost for text, doubles the HTML.
Server-render the saved locale, islands re-render on toggle. Smaller HTML; every island then needs both strings client-side anyway.
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

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