osCommerce Language File Issue

Working with an osCommerce shop, I periodically hit a ‘503 Server Error’.

Found the following messages in the logs:

PHP Warning: require(includes/languages/.php): failed to open stream: No such file or directory …  
PHP Fatal error: require(): Failed opening required 'includes/languages/.php'

To get it working you need to edit the file a bit:
includes/application_top.php

Find this line:

// set the language
if (!tep_session_is_registered(language) || isset($HTTP_GET_VARS['language'])) {

Change it to look like this:

// set the language
if (!tep_session_is_registered(language) || isset($HTTP_GET_VARS['language']) || empty($language)) {