Steps to reproduce the issue

Install Joomla! 3.8.8 with some sample data
Enable plugin "System - Language Filter"
Set error reporting to maximum

Expected result

No undefined index notice when we have bad or malformed user agent.

Actual result

undefined index notice

System information (as much as possible)

Web Server Apache
PHP Version 7.0.27-0+deb9u1
Database Version 5.5.5-10.1.26-MariaDB-0+deb9u1

Additional comments

When I request home page with the following user agent "Mozilla/5.0 (Windows NT 5.1; U; de) Opera 8.50"
I get the error (in php errors log file)
PHP Notice: Undefined offset: 1 in /home/joomla_root/libraries/vendor/joomla/application/src/Web/WebClient.php on line 424
The same notice can happen with other (fake or old) user agents, see the code above in mentioned source file.

command to check
wget -U "Mozilla/5.0 (Windows NT 5.1; U; de) Opera 8.50" http://my.site/