The default Moodle installation includes many Language packs. A language pack is a set of translations for the Moodle interface. Language packs translate the Moodle interface, and not the course content. Here's the Front Page of a site when the user selects Spanish from the language menu:

Note that every aspect of the interface is being presented in Spanish: menu names, menu items, section names, buttons, and system messages. Now, let's take a look at the same Front Page when the user selects Romanian from the language menu:

Note that much of the interface has not been translated. For example, the Site Administration menu and section name for Site news are still in English. When a part of Moodle's interface is not translated into the selected language, Moodle uses the English version.