Index: user.module
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.module,v
retrieving revision 1.1055
diff -u -r1.1055 user.module
--- modules/user/user.module	2 Oct 2009 14:49:10 -0000	1.1055
+++ modules/user/user.module	3 Oct 2009 06:32:21 -0000
@@ -3038,7 +3038,7 @@
  * which is why it can't be in the user.pages.inc file.
  */
 function user_register_submit($form, &$form_state) {
-  global $base_url;
+  global $base_url, $language;
   $admin = user_access('administer users');
 
   $mail = $form_state['values']['mail'];
@@ -3069,6 +3069,11 @@
   unset($form_state['values']['form_token'], $form_state['values']['submit'], $form_state['values']['op'], $form_state['values']['notify'], $form_state['values']['form_id'], $form_state['values']['affiliates'], $form_state['values']['destination'], $form_state['values']['form_build_id']);
 
   $merge_data = array('pass' => $pass, 'init' => $mail, 'roles' => $roles);
+
+  if (empty($form_state['values']['language'])) {
+    $merge_data['language'] = $language->language;
+  }
+
   if (!$admin) {
     // Set the user's status because it was not displayed in the form.
     $merge_data['status'] = variable_get('user_register', 1) == 1;
Index: user.test
===================================================================
RCS file: /cvs/drupal/drupal/modules/user/user.test,v
retrieving revision 1.62
diff -u -r1.62 user.test
--- modules/user/user.test	19 Sep 2009 10:54:35 -0000	1.62
+++ modules/user/user.test	3 Oct 2009 06:33:18 -0000
@@ -123,7 +123,7 @@
     $this->assertTrue(($new_user->created > REQUEST_TIME - 20 ), t('Correct creation time.'));
     $this->assertEqual($new_user->status, variable_get('user_register', 1) == 1 ? 1 : 0, t('Correct status field.'));
     $this->assertEqual($new_user->timezone, variable_get('date_default_timezone'), t('Correct time zone field.'));
-    $this->assertEqual($new_user->language, '', t('Correct language field.'));
+    $this->assertEqual($new_user->language, $GLOBALS['language']->language, t('Correct language field.'));
     $this->assertEqual($new_user->picture, '', t('Correct picture field.'));
     $this->assertEqual($new_user->init, $mail, t('Correct init field.'));
   }