system/Config/Services.php
public static function negotiator(RequestInterface $request = null, bool $getShared = true)
	{
		if ($getShared)
		{
			return static::getSharedInstance('negotiator', $request);
		}

		if (is_null($request))
		{
			$request = static::request();
		}

		return new Negotiate($request);
	}
public static function negotiator(RequestInterface $request = null, bool $getShared = true)
	{
		if ($getShared)
		{
			return static::getSharedInstance('negotiator', $request);
		}

		if (is_null($request))
		{
			$request = static::request();
		}

		return new Negotiate($request);
	}
The Negotiate class provides the content negotiation features for
working the request to determine correct language, encoding, charset,
and more.

@param \CodeIgniter\HTTP\RequestInterface $request
@param boolean                            $getShared

@return \CodeIgniter\HTTP\Negotiate