framework/i18n/MessageSource.php
public function init()
    {
        parent::init();
        if ($this->sourceLanguage === null) {
            $this->sourceLanguage = Yii::$app->sourceLanguage;
        }
    }
public function init()
    {
        parent::init();
        if ($this->sourceLanguage === null) {
            $this->sourceLanguage = Yii::$app->sourceLanguage;
        }
    }
Initializes this component.