Localization within pimcore is very easy, because pimcore follows 100% the ZF patterns.
Setting up the languages
First of all you have to define the languages which should be available in your pimcore installation, you can configure that in Settings -> System -> Website:

This languages will be available in various modules within pimcore like :
- Document - Localization (system property for language)
- Localized Fields for Objects (object localization)
- Translations (Zend_Translate)
- ...
How pimcore deals with locales
pimcore offers localization for documents, if you don't want to use that, you can also register the locale manually anywhere in your code.
You can do that simply how you would do that in every other ZF application:
$locale = new Zend_Locale("en_US"); Zend_Registry::set("Zend_Locale",$locale);
Now every pimcore and ZF module will respect your registered locale.