code: public function negotiate()
      {
          $request = $this->request ?: Yii::$app->getRequest();
          $response = $this->response ?: Yii::$app->getResponse();
          if (!empty($this->formats)) {
              if (\count($this->formats) > 1) {
                  $response->getHeaders()->add('Vary', 'Accept');
              }
              $this->negotiateContentType($request, $response);
          }
          if (!empty($this->languages)) {
              if (\count($this->languages) > 1) {
                  $response->getHeaders()->add('Vary', 'Accept-Language');
              }
              Yii::$app->language = $this->negotiateLanguage($request);
          }
      }
docs: Negotiates the response format and application language.