uses a link to 1.0.12 language explanation.
Language - Joomla! Extensions Directory Languages
New to Joomla! 1.5 and perhaps the most basic and critical extension is a language. Languages are packaged as either a core language pack or an extension language pack. They allow both the Joomla! core as well as third party components and modules to be internationalised.
More information on languages is available from the Joomla! help manual: Languages