Index: i18ntaxonomy/i18ntaxonomy.js
===================================================================
--- i18ntaxonomy/i18ntaxonomy.js	(revision 0)
+++ i18ntaxonomy/i18ntaxonomy.js	(revision 0)
@@ -0,0 +1,16 @@
+Drupal.behaviors.i18ntaxonomy = function (context) {
+  if (Drupal.settings && Drupal.settings.i18ntaxonomy_vocabulary_form) {
+    $('form#taxonomy-form-vocabulary input.form-radio', context).filter('[name=i18nmode]').click(function () {
+      var languageSelect = $('form#taxonomy-form-vocabulary select#edit-language', context);
+      if ($(this).val() == Drupal.settings.i18ntaxonomy_vocabulary_form.I18N_TAXONOMY_LANGUAGE) {
+        // Make sure language form is enabled when I18N_TAXONOMY_LANGUAGE is clicked
+        languageSelect.removeAttr("disabled");
+      }
+      else {
+        // Make sure language form is disabled otherwise and set to blank.
+        languageSelect.val("");
+        languageSelect.attr("disabled", "disabled");
+      }
+    });
+  }
+};
Index: i18ntaxonomy/i18ntaxonomy.module
===================================================================
--- i18ntaxonomy/i18ntaxonomy.module	(revision 22167)
+++ i18ntaxonomy/i18ntaxonomy.module	(working copy)
@@ -347,6 +347,8 @@
         $vocabulary = NULL;
         $mode =  I18N_TAXONOMY_NONE;
       }
+      drupal_add_js(drupal_get_path('module', 'i18ntaxonomy') . '/i18ntaxonomy.js');
+      drupal_add_js(array('i18ntaxonomy_vocabulary_form' => array('I18N_TAXONOMY_LANGUAGE' => I18N_TAXONOMY_LANGUAGE)), 'setting');
       $form['i18n'] = array(
         '#type' => 'fieldset',
         '#title' => t('Multilingual options'),
@@ -368,6 +370,7 @@
         '#description' => t('Language for this vocabulary. If set, it will apply to all terms in this vocabulary.'),
         '#disabled' => ($vocabulary && $mode != I18N_TAXONOMY_LANGUAGE),
       );
+      $form['#validate'][] = 'i18ntaxonomy_form_vocabulary_validate';
       break;
 
     case 'taxonomy_form_term': // Taxonomy term
@@ -433,6 +436,13 @@
   }
 }
 
+function i18ntaxonomy_form_vocabulary_validate($form, &$form_state) {
+  $language = isset($form_state['values']['language']) ? $form_state['values']['language'] : '';
+  if ($form_state['values']['i18nmode'] != I18N_TAXONOMY_LANGUAGE && $language) {
+    form_set_error('language', t('Setting a vocabulary language only makes sense in the "Set language to vocabulary" translation mode. Either change to this mode or do not select a language.'));
+  }
+}
+
 /**
  * Localize a taxonomy_form_all() kind of control
  *