Language - How to structure a multilanguage site?

[eluser]Mauricio de Abreu Antunes[/eluser]
I'm coding a web site. He has 3 language choices: spanish, portuguese and english.
In CI user guide, it tells me to put my translations in application/language.
Ok, i did it. But i wanna translate form validation errors, like required or is_unique.
How do to this? How to avoid system/core/language/english files?

Thanks, bye!