You can use the class variables to achieve that, here's an example:
<?php
class ApiController extends Controller
{
public $lang = 'en';
public function beforeAction($action)
{
if(Yii::app()->session->contains('lang'))
$this->lang = Yii::app()->session['lang'];
return parent::beforeAction($action);
}
public function afterAction($action,$params)
{
Yii::app()->session['lang'] = $this->lang;
return parent::afterAction($action,$params);
}
public function actionDisplay(){
echo 'In Display action';
$this->lang = 'test'
echo $this->lang;
}
public function actionDisplay2(){
echo 'In Display2 action';
echo $this->lang;
}
}