Constructor, creates a PoHeader with default values.
string $langcode: Language code.
public function __construct($langcode = NULL) {
$this->_langcode = $langcode;
// Ignore errors when run during site installation before
// date_default_timezone_set() is called.
$this->_po_date = @date("Y-m-d H:iO");
$this->_pluralForms = 'nplurals=2; plural=(n > 1);';
}