First of all, you should download the language pack which you wish to install. You can find a lot of language packs at this URL.

Once you have downloaded the language pack, you can install it from your Joomla 1.5 admin area > Extensions > Install/Uninstall.

Then you can set the newly-installed language as your Joomla's default from Extensions > Language Manager. Once there, simply check the box next to it and then click the Default button located at the top-right Joomla menu.

If you have problems installing a new language in Joomla 1.5, ask your host for assistance. SiteGround provides the best Joomla hosting and such requests are handled with ease.