CodeIgniter has been translated to many international languages, not only the User Guide but also the messages and errors strings in its language packs.

For example, you can find Spanish translations for your CodeIgniter applications, here. [Make sure to download the right version, ie: 1.7.2]

The next step once you unpack the .zip is to copy the files into your language directory that usually is located under /system/languages so it will read /system/languages/spanish

Then, to use Spanish messages in your applications you’ll need to chaneg the default language setting in /application/config/config.php under line $config['language']