test(12-01): add failing tests for navigator.language auto-detection

- test_navigator_language_en_sets_lang_en: expects lang='en' when locale=en-US and localStorage empty
- test_navigator_language_fr_sets_lang_fr: expects lang='fr' when locale=fr-FR and localStorage empty
- test_localstorage_wins_over_navigator_language: expects localStorage to override navigator.language

tests/e2e/test_i18n_toggle.py

@@ -3,6 +3,79 @@ from __future__ import annotations
 import pytest
 
 
+# ---------------------------------------------------------------------------
+# Task 1 (Phase 12-01): navigator.language auto-detection
+# ---------------------------------------------------------------------------
+
+def test_navigator_language_en_sets_lang_en(browser, live_server: str) -> None:
+    """When localStorage is empty and navigator.language is 'en-US', lang defaults to 'en'."""
+    context = browser.new_context(locale="en-US")
+    page = context.new_page()
+
+    # Clear any saved preference so localStorage fallback is not used
+    page.goto(f"{live_server}/", wait_until="domcontentloaded")
+    page.evaluate("localStorage.removeItem('imptune_lang')")
+
+    # Reload so the alpine:init IIFE runs with no localStorage and en-US locale
+    page.reload(wait_until="domcontentloaded")
+    page.wait_for_function(
+        "document.querySelector('nav a[href=\"/printers\"]').textContent.trim() !== ''",
+        timeout=3000,
+    )
+
+    lang = page.evaluate("Alpine.store('i18n').lang")
+    assert lang == "en", f"Expected 'en' when navigator.language is 'en-US', got '{lang}'"
+
+    context.close()
+
+
+def test_navigator_language_fr_sets_lang_fr(browser, live_server: str) -> None:
+    """When localStorage is empty and navigator.language is 'fr-FR', lang defaults to 'fr'."""
+    context = browser.new_context(locale="fr-FR")
+    page = context.new_page()
+
+    page.goto(f"{live_server}/", wait_until="domcontentloaded")
+    page.evaluate("localStorage.removeItem('imptune_lang')")
+
+    page.reload(wait_until="domcontentloaded")
+    page.wait_for_function(
+        "document.querySelector('nav a[href=\"/printers\"]').textContent.trim() !== ''",
+        timeout=3000,
+    )
+
+    lang = page.evaluate("Alpine.store('i18n').lang")
+    assert lang == "fr", f"Expected 'fr' when navigator.language is 'fr-FR', got '{lang}'"
+
+    context.close()
+
+
+def test_localstorage_wins_over_navigator_language(browser, live_server: str) -> None:
+    """localStorage preference wins over navigator.language when a value is saved."""
+    # Browser locale is 'en-US' but localStorage says 'fr' → should use 'fr'
+    context = browser.new_context(locale="en-US")
+    page = context.new_page()
+
+    page.goto(f"{live_server}/", wait_until="domcontentloaded")
+    page.evaluate("localStorage.setItem('imptune_lang', 'fr')")
+
+    page.reload(wait_until="domcontentloaded")
+    page.wait_for_function(
+        "document.querySelector('nav a[href=\"/printers\"]').textContent.trim() !== ''",
+        timeout=3000,
+    )
+
+    lang = page.evaluate("Alpine.store('i18n').lang")
+    assert lang == "fr", f"Expected localStorage 'fr' to win over navigator.language 'en-US', got '{lang}'"
+
+    # Cleanup
+    page.evaluate("localStorage.removeItem('imptune_lang')")
+    context.close()
+
+
+# ---------------------------------------------------------------------------
+# Original toggle and persistence tests
+# ---------------------------------------------------------------------------
+
 def test_language_toggle_switches_nav_label(page, live_server: str) -> None:
     """Clicking FR/EN button switches nav label from French to English."""
     page.goto(f"{live_server}/", wait_until="domcontentloaded")