framework/filters/ContentNegotiator.php
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);
        }
    }
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);
        }
    }
Negotiates the response format and application language.