In Joomla! 1.5 all User interfaces can be localised. This includes the installation, the Back-end Control Panel and the Front-end Site.
The core release of Joomla! 1.5 is shipped with multiple language choices in the installation but, other than English (the default), languages for the Site and Administration interfaces need to be added after installation. Links to such language packs exist below.
Translation Teams for Joomla! 1.5 may have also released fully localised installation packages where site, administrator and sample data are in the local language. These localised releases can be found in the specific team projects on the Joomla! Extensions Directory.
How do I install language packs?
How do I select languages?
Where can I find Language Packs and Localised Releases?
Please note that Joomla! 1.5 is new and language packs for this version may have not been released at this time.