Internationalization, or running a website in multiple languages, is a complicated problem with several parts:

1. The application itself. Your toolbar, the editing window - all the interface you use to run concrete5 can be translated into different languages. If you've got administrators who don't speak english, you may want to run concrete5 in a different language. You can find language packs from the developers internationalization area. Once you have them installed, you can choose which language is default from this interface. 
    
2. The content of your website. If you want to run a website with content in multiple langauges, you need the free Internationalization add-on. If you have a group of translators involved and you want to have some process management tools, you may want the Enterprise Internationalization add-on.