Unpack the zip and upload the contents of the /xtcommerce directory to your desired server location, then start the install by typing in
http://www.your domain/your directory/xtc_installer
Install is in English or German
A list of directories the may have to be CHMODed will also appear. Change these per ftp immediately before continuing.
The out-of-the-box install is German/English anyway so no problems there. If you don't need the German then deinstall it in the admin after installation.
Hope that helps