If lang.class.php doesn't actually echo anything to the user (and, in my opinion, it shouldn't if it's just a class definition file), then look for whitespace outside of your <?php and ?> tags.
99% of all "Cannot add header info" errors are due to whitespace outside of the PHP delimiter tags. This whitespace is considered HTML by PHP, so if PHP encounters any, it'll send the default HTTP headers and begin the HTML document with that whitespace.
Take care,
Nik
http://www.bigaction.org/