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, not the course content.
Administration → Site Administration → Language
However, some of the interface is not translated. When a part of Moodle's interface is not translated into the selected language, Moodle uses the English version.
When you install an additional language, Moodle places the language pack in its home directory, under the sub directory /lang. It creates a subdirectory for each language's files. Inside the directory of a language pack, you can see a list of files that contain the translations.