add support for translating placeholders

customize.dist/messages.js

@@ -79,10 +79,16 @@ define(['/customize/languageSelector.js',
         var key = $el.data('localization-title');
         $el.attr('title', messages[key]);
     };
+    var translatePlaceholder = function (i, e) {
+        var $el = $(this);
+        var key = $el.data('localization-placeholder');
+        $el.attr('placeholder', messages[key]);
+    };
     messages._applyTranslation = function () {
         $('[data-localization]').each(translateText);
         $('#pad-iframe').contents().find('[data-localization]').each(translateText);
         $('[data-localization-title]').each(translateTitle);
+        $('[data-localization-placeholder').each(translatePlaceholder);
         $('#pad-iframe').contents().find('[data-localization-title]').each(translateTitle);
     };