Due to space restrictions, the official Ubuntu installation CDs (also known as ISO images) that can be downloaded from ubuntu.com contain only a handful of the many languages in which Ubuntu is available. Any additional languages can then be downloaded during or after the installation.
Localized images are customized versions of the original media, which are modified to include the translation of a particular language or other modifications that are relevant to a particular location. The tools used to create them were designed as part of the desktop-o-cd-localization spec.
They make it possible to have native language support upon installation and without the need of a working Internet connection. In addition to the language, they also enable customizing other aspects of Ubuntu to make it more relevant to a particular culture or region.
This document will collect all the information related to creating localized images.
How to create localized images
This Ask Ubuntu question describes the step-by-step procedure to create localized images.
Which policies apply to localized images
Testing localized images