Step 1: you need download the language pack which you wish to install. You can find a lot of language packs at this URL.
Step 2: Login to Joomla admin area
Step 3: Go to Extensions and then Install/Uninstall.
Step 4: Then you can set the newly-installed language as your Joomla’s default from Extensions > Language Manager.
Step 5: Simply check the box next to it and then click the Default button located at the top-right Joomla menu.