Index: includes/language.inc
===================================================================
RCS file: /cvs/drupal/drupal/includes/language.inc,v
retrieving revision 1.19
diff -u -p -r1.19 language.inc
--- includes/language.inc	1 Feb 2009 16:45:53 -0000	1.19
+++ includes/language.inc	27 Mar 2009 12:06:35 -0000
@@ -59,6 +59,11 @@ function language_initialize() {
 
   // Browser accept-language parsing.
   if ($language = language_from_browser()) {
+    // If the language is set from  browser preferences, set CACHE_DISABLED
+    // to avoid caching the page in this language for subsequent requests.
+    if ($language != language_default()) {
+      $GLOBALS['conf']['cache'] = CACHE_DISABLED;
+    }
     return $language;
   }
 
Index: modules/simpletest/tests/bootstrap.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/simpletest/tests/bootstrap.test,v
retrieving revision 1.12
diff -u -p -r1.12 bootstrap.test
--- modules/simpletest/tests/bootstrap.test	31 Jan 2009 16:50:57 -0000	1.12
+++ modules/simpletest/tests/bootstrap.test	27 Mar 2009 12:06:35 -0000
@@ -80,6 +80,75 @@ class BootstrapIPAddressTestCase extends
   }
 }
 
+/**
+ * Test page caching with language negotiation.
+ */
+class BootstrapPageCacheWithLanguageFallbackTestCase extends DrupalWebTestCase {
+
+  function getInfo() {
+    return array(
+      'name' => t('Page cache test with language negotiation'),
+      'description' => t('Enable the page cache and language negotiation, ensure that cached pages are served in the correct language.'),
+      'group' => t('Bootstrap')
+    );
+  }
+
+  function setUp() {
+    parent::setUp('locale');
+    $this->web_user = $this->drupalCreateUser(array('administer languages', 'access administration pages'));
+    $this->drupalLogin($this->web_user);
+
+    // Enable French language.
+    $edit = array();
+    $edit['langcode'] = 'fr';
+
+    $this->drupalPost('admin/settings/language/add', $edit, t('Add language'));
+    $this->drupalLogout($this->web_user);
+
+    // Set language negotiation to "Path prefix with fallback".
+    variable_set('language_negotiation', LANGUAGE_NEGOTIATION_PATH);
+
+    // Force inclusion of language.inc.
+    drupal_init_language();
+    // Enable the page cache.
+    variable_set('cache', CACHE_NORMAL);
+  }
+
+  /**
+   * Test page caching with language fallback.
+   */
+  function testPageCacheLanguageFallback() {
+
+    // Visit the root path with browser language preference set to French.
+    // Since this is in the default site language (English), the page should
+    // not be cached for this request.
+    $this->drupalGet('', array(), array('Accept-Language: fr'));
+
+    // The visitor should be served the page in their preferred language.
+    $this->assertRaw('xml:lang="fr"', t('Page returned in the correct language.'));
+
+    // Since the page has been set by language preferences, we should neither
+    // serve the page from cache nor cache the rendered page..
+    $this->assertFalse(cache_get(url('', array('absolute' => TRUE)), 'cache_page'), t('Page has not been cached.'));
+    $this->assertFalse($this->drupalGetHeader('Etag'), t('Page not served from cache.'));
+
+    // Visit the page again with no browser language preference, and confirm
+    // that it is returned in the correct language.
+    $this->drupalGet('');
+    $this->assertRaw('xml:lang="en"', t('Page returned in the correct language.'));
+    // This request should not have been served from cache since there have
+    // been no valid requests.
+    $this->assertFalse($this->drupalGetHeader('Etag'), t('Page not served from cache.'));
+    // Confirm the cache was populated from this request.
+    $this->assertTrue(cache_get(url('', array('absolute' => TRUE)), 'cache_page'), t('Page has been cached.'));
+
+    // Ensure that the page served from cache correctly when requested again.
+    $this->drupalGet('');
+    $this->assertTrue($this->drupalGetHeader('Etag'), t('Page served from cache'));
+    $this->assertRaw('xml:lang="en"', t('Page returned in the correct language.'));
+  }
+}
+
 class BootstrapPageCacheTestCase extends DrupalWebTestCase {
 
   function getInfo() {