added constant for every language

lang/UTF-8/serendipity_lang_fr.inc.php

@@ -980,3 +980,4 @@
 
 @define('ARCHIVE_SORT_STABLE', 'Stable Archives');
 @define('ARCHIVE_SORT_STABLE_DESC', 'Sort the archive-pages descending, so they are stable and search-crawler do not have to reindex them.');
+@define('PLAIN_ASCII_NAMES', '(no special characters, umlauts)');