Add check for "plugins" directory writability

lang/serendipity_lang_tw.inc.php

@@ -950,3 +950,4 @@ $i18n_unknown = 'tw';
 
 @define('SYNDICATION_PLUGIN_FEEDBURNERID_FORWARD2', 'If you set this option to "Force" you can forward the RSS feed to any webservice, not only FeedBurner. Look at the option "Feedburner ID" below to enter an absolute URL)');
 @define('COMMENTS_FILTER_NEED_CONFIRM', 'Pending user confirmation');
+@define('NOT_WRITABLE_SPARTACUS', ' (Only required when you plan to use Spartacus plugin for remote plugin download)');