You have to download the language packs you want to install. They can be obtained from the following url:

http://www.phpbbhacks.com/category/15/2

All you have to do is to upload the files into the languages/ phpBB2 directory on the server, and it will be available automatically in the drop-down list in the user profile. You can do this using your favorite FTP client. We have a nice tutorial on FTP clients that might be helpful to you locate on the following link:

http://www.siteground.com/ftp_client_account.htm

You can also set the default language for the board in the admin control panel. Please note that you should always keep the English language pack in place as it is the default one.