Switch system language

I'm using CodeIgniter 3.0. I'm writing a multilingual website. The website is not text-heavy, so it was not much work using the Language class and the documentation on the internet is plenty. However, what I was unable to find is documentation on switching the system language.

In config.php I have set the language to 'english'. This means that if my user sets a different language the system messages, e.g., from form validation, are still in English. How can I switch the language for those type of messages?