The first step you must download the language pack of your choice. You can visit the Joomla Extentions - Languages page.
Then, you enter the administration panel. Chose Installers menu and select Languages.

Press the [Browse] button to locate the installation pack for your language...



and then select the [Upload File & Install].
Wait for the confirmation and press the Continue.. link. From the listed languages select the new language and then click the [Publish] icon.


Now you have successfully installed a new language pack on your Joomla system.

Next we will set it as default. For this purpose please go to Joomla's admin panel: Global Configuration -> Site, the Locale tab. In the Language option choose the newly installed one and this will automatically set it as default for your site.
Well done! You have successfully changed the language the default language of your Joomla!