3.9.4. Error: Failed opening 'language/lang-.php' for inclusion

You either

• didn't install the language files lang-xxx.php (like language/lang-english.php), or

• you cannot connect to the database, so that the value of the default language could not be set and is empty (and that's why you get an error about lang-.php, instead of lang-english.php, or lang-french.php etc.). Try the small test script (see Call to undefined function: message die in db.php line 88):

  <?php include("config.php"); mysql_connect("$dbhost", "$dbuname", "$dbpass"); mysql_select_db("$dbname"); echo mysql_error(); phpinfo(); ?>

  (see also Section 3.9.1), or

• your include path information is missing some paths (like ".", the current directory), see Section 3.9.10.