framework/i18n/I18N.php
public function init()
    {
        parent::init();
        if (!isset($this->translations['yii']) && !isset($this->translations['yii*'])) {
            $this->translations['yii'] = [
                'class' => 'yii\i18n\PhpMessageSource',
                'sourceLanguage' => 'en-US',
                'basePath' => '@yii/messages',
            ];
        }

        if (!isset($this->translations['app']) && !isset($this->translations['app*'])) {
            $this->translations['app'] = [
                'class' => 'yii\i18n\PhpMessageSource',
                'sourceLanguage' => Yii::$app->sourceLanguage,
                'basePath' => '@app/messages',
            ];
        }
    }
public function init()
    {
        parent::init();
        if (!isset($this->translations['yii']) && !isset($this->translations['yii*'])) {
            $this->translations['yii'] = [
                'class' => 'yii\i18n\PhpMessageSource',
                'sourceLanguage' => 'en-US',
                'basePath' => '@yii/messages',
            ];
        }

        if (!isset($this->translations['app']) && !isset($this->translations['app*'])) {
            $this->translations['app'] = [
                'class' => 'yii\i18n\PhpMessageSource',
                'sourceLanguage' => Yii::$app->sourceLanguage,
                'basePath' => '@app/messages',
            ];
        }
    }
Initializes the component by configuring the default message categories.