core-bundle/src/EventListener/LocaleListener.php
private function getLocale(Request $request): string
    {
        if (null !== $request->attributes->get('_locale')) {
            return $this->formatLocaleId($request->attributes->get('_locale'));
        }

        return $request->getPreferredLanguage($this->availableLocales);
    }
private function getLocale(Request $request): string
    {
        if (null !== $request->attributes->get('_locale')) {
            return $this->formatLocaleId($request->attributes->get('_locale'));
        }

        return $request->getPreferredLanguage($this->availableLocales);
    }
Returns the locale from the request, the session or the HTTP header.