New to Joomla! 1.5 and perhaps the most basic and critical Extension is a Language. Joomla! is released with multiple Installation Languages but the base Site and Administrator are packaged in just the one Language en-GB - being English with GB spelling for example. To include all the translations currently available would bloat the core package and make it unmanageable for uploading purposes. The Language files enable all the User interfaces both Front-end and Back-end to be presented in the local preferred language. Note these packs do not have any impact on the actual content such as Articles.
More information on languages is available from the Joomla! help site: Language Packs and Localised Releases