$session = Yii::$app->session;
$session->open();
$session->set('language', 'ru');
print_r($session);
yii\web\Session#1
(
[flashParam] => '__flash'
[handler] => null
[yii\web\Session:_cookieParams] => [
'httponly' => true
]
[yii\web\Session:_hasSessionId] => null
[yii\base\Component:_events] => []
[yii\base\Component:_behaviors] => null
)