framework/validators/RequiredValidator.php
public function init()
    {
        parent::init();
        if ($this->message === null) {
            $this->message = $this->requiredValue === null ? Yii::t('yii', '{attribute} cannot be blank.')
                : Yii::t('yii', '{attribute} must be "{requiredValue}".');
        }
    }
public function init()
    {
        parent::init();
        if ($this->message === null) {
            $this->message = $this->requiredValue === null ? Yii::t('yii', '{attribute} cannot be blank.')
                : Yii::t('yii', '{attribute} must be "{requiredValue}".');
        }
    }
{@inheritdoc}