diff --git a/.agents/context/content-i18n.md b/.agents/context/content-i18n.md
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+# Context: full-guide language switching
+
+## Decision
+
+The Astro full guide keeps the current client-side language switch on its
+existing `/full-guide/` URL. It server-renders both locale variants and the
+language-toggle island shows the selected variant after `client:idle` hydration.
+
+This deliberately preserves the current no-URL-change contract, including links
+shared without a locale segment, and avoids a route/redirect and publishing
+change. The cost is duplicated localized HTML and both locales in the response.
+That is acceptable for this small guide and avoids sending duplicated string
+data through every interactive island.
+
+## Markup and island contract for task 15d
+
+- Render each static localized fragment twice. Put `data-language-content="en"`
+  or `data-language-content="pt"` on its outer element. English is visible in
+  server HTML; the toggle uses the native `hidden` attribute for the inactive
+  locale.
+- Add `<LanguageToggle />` to the guide top bar. Astro's `client:idle` directive
+  is only valid for framework components; this `.astro` island defers its
+  browser setup with `requestIdleCallback` (and a timeout fallback) instead. Do
+  not hydrate the page or use `client:load`; the control is deliberately
+  idle-priority.
+- The island owns the `ai-for-dummies-language` localStorage key. Every read and
+  write remains inside `try`/`catch`, because previews may disable storage.
+- On each selection the island sets `<html lang>` to `en` or `pt-BR`, updates
+  its `[data-lang]` buttons' `.active` class and `aria-pressed` state, updates
+  `[data-language-content]`, then dispatches `ai-for-dummies:languagechange` on
+  `window`. The event detail is `{ language: 'en' | 'pt' }`.
+- The guide selector island (15a) must read `document.documentElement.lang` when
+  it hydrates and listen for that event. On receipt it must re-render the
+  currently active phase and all active selector panels from their collection
+  data. This preserves today’s `applyLanguage` behaviour without coupling the
+  toggle to page selectors.
+
+This is a page-local contract: the existing `/rules/` toggle continues using its
+own `rules-language` key and must not be changed as part of full-guide
+migration.
diff --git a/src/components/islands/LanguageToggle.astro b/src/components/islands/LanguageToggle.astro
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+---
+// LanguageToggle — idle-hydrated, full-guide-only locale control.
+//
+// The guide server-renders both locales. This leaf owns the persisted locale
+// and announces changes so selector islands can rebuild their active panels
+// from collection data without knowing about this control's markup.
+---
+
+<div class="lang-switch" aria-label="Language" data-language-toggle>
+  <button class="active" data-lang="en" aria-pressed="true">EN</button>
+  <span>/</span>
+  <button data-lang="pt" aria-pressed="false">PT</button>
+</div>
+
+<script>
+  type Language = 'en' | 'pt';
+
+  function setup(root: HTMLElement) {
+    const buttons = root.querySelectorAll<HTMLButtonElement>('[data-lang]');
+
+    function renderLanguage(next: Language) {
+      document.documentElement.lang = next === 'pt' ? 'pt-BR' : 'en';
+      buttons.forEach((button) => {
+        const isActive = button.dataset.lang === next;
+        button.classList.toggle('active', isActive);
+        button.setAttribute('aria-pressed', String(isActive));
+      });
+      document.querySelectorAll<HTMLElement>('[data-language-content]').forEach((node) => {
+        node.hidden = node.dataset.languageContent !== next;
+      });
+      window.dispatchEvent(
+        new CustomEvent('ai-for-dummies:languagechange', { detail: { language: next } }),
+      );
+    }
+
+    function selectLanguage(value: string | undefined): Language {
+      return value === 'pt' ? 'pt' : 'en';
+    }
+
+    buttons.forEach((button) => {
+      button.addEventListener('click', () => {
+        const language = selectLanguage(button.dataset.lang);
+        renderLanguage(language);
+        try {
+          localStorage.setItem('ai-for-dummies-language', language);
+        } catch {
+          // Preview environments can disable storage.
+        }
+      });
+    });
+
+    let savedLanguage = 'en';
+    try {
+      savedLanguage = localStorage.getItem('ai-for-dummies-language') || 'en';
+    } catch {
+      // Preview environments can disable storage.
+    }
+    renderLanguage(selectLanguage(savedLanguage));
+  }
+
+  document.querySelectorAll<HTMLElement>('[data-language-toggle]').forEach((root) => {
+    if ('requestIdleCallback' in window) {
+      window.requestIdleCallback(() => setup(root));
+    } else {
+      setTimeout(() => setup(root), 0);
+    }
+  });
+</script>
