Quick fix for this issue (new version will be released soon):
1. Open file /plugins/editors-xtd/jcommentson.php
2. Replace line:
$this->loadLanguage();
with
$this->loadLanguage('plg_editors-xtd_jcommentson', JPATH_ADMINISTRATOR);
3. Open file /plugins/editors-xtd/jcommentsoff.php
4. Replace line:
$this->loadLanguage();
with
$this->loadLanguage('plg_editors-xtd_jcommentsoff', JPATH_ADMINISTRATOR);