Problems installing multiple languages

I feel I am missing something obvious, but I can't seem to find how to install other language packs via this plugin the way it seems I am supposed to. I can find the language files at wordpress.org, but I gather I am supposed to be able to install language files via FTP to my site, yet there are no language files in the downloaded plugin. The only two files in the /languages directory are signup_language-default.po and signup_language.pot. I hope to install this on my main network but right now it's on a test network. Signing up for a new account on this network only allows me to select English as the site language. I don't want to change the default language for my whole network--just give users the options to have sites in their own language. What am I doing wrong?