By default, Joomla language is English. You can download language that you want to use on your site. In a few steps we will show how to install language pack.
STEP 1.
Download from extension.joomla.org language pack that you want to install to your site.

STEP 2.
In the Joomla 1.5 admin panel go to Extensions – Install / Unistall

STEP 3.
In the Upload Package File go to Browse and select the language on your computer that you want to install and then click on Upload file & Install. In our example it is Serbian language.

STEP 4.
In the Joomla 1.5 admin panel go to Extensions – Language Manager

STEP 5.
Select Language Name and make it Default