① Download
② Extract content in your Computer
③ Using a FTP Client (Filezilla, SmartFTP) upload the folder /INCLUDE to the root of your phpFox´s installation
④ Go to AdminCP > Extensions > Language > Import Language… and wait for the process to end
⑤ (Optional) Go to AdminCP > Tools > Maintenance > Manage Cache… and press Clear Cache

Done! now you can set the default language of your site to SPANISH