Added CENTER constant

lang/UTF-8/serendipity_lang_pl.inc.php

@@ -897,3 +897,4 @@ $i18n_filename_to   = array('_', 'a', 'A', 'a', 'A', 'b', 'B', 'c', 'C', 'c', 'C
 @define('RSS_IMPORT_WPXRSS', 'WordPress eXtended RSS import, requires PHP5 and might take up much memory');
 @define('SET_TO_MODERATED', 'Moderate');
 @define('COMMENT_MODERATED', 'Comment #%s has successfully been set as moderated');
+@define('CENTER', 'center');