Language code in header.tpl

Hi
I use Opencart 2.3 and i need variable with language code in header.tpl file.
I use this code (I found this code in this forum)

echo $this->session->data['language'];

but it causes an error.

<b>Notice</b>: Undefined property: Template\PHP::$session in <b>C:\xampp7\htdocs\immunology-shop\catalog\view\theme\immunology\template\common\header.tpl</b> on line <b>102</b><b>Notice</b>: Trying to get property of non-object in <b>C:\xampp7\htdocs\immunology-shop\catalog\view\theme\immunology\template\common\header.tpl</b> on line <b>102</b>

Somebody help me? Please

Template files are outside the scope of that variable (should be worth reading about if you want to develop anything), if you want to use it you need to declare it in the respective controller and then use it in the template, like so.

controller
template

Ok I paste in controller header code:
$data['language'] = $this->session->data['language'];
and
code echo $language;
in header.tpl not work.

Are You sure. It not work for me?