Fixed language string in mailman on CentOS 7.3

Author: Till Brehm

install/lib/installer_base.lib.php

@@ -777,7 +777,7 @@ public function configure_mailman($status = 'insert') {
 			$virtual_domains = "' '";
 
 		$content = str_replace('{hostname}', $conf['hostname'], $content);
-		if(!isset($old_options['DEFAULT_SERVER_LANGUAGE']) || $old_options['DEFAULT_SERVER_LANGUAGE'] == '') $old_options['DEFAULT_SERVER_LANGUAGE'] = 'en';
+		if(!isset($old_options['DEFAULT_SERVER_LANGUAGE']) || $old_options['DEFAULT_SERVER_LANGUAGE'] == '') $old_options['DEFAULT_SERVER_LANGUAGE'] = "'en'";
 		$content = str_replace('{default_language}', $old_options['DEFAULT_SERVER_LANGUAGE'], $content);
 		$content = str_replace('{virtual_domains}', $virtual_domains, $content);