qTranslate, WPML and Polylang are, in this respect, working the same way. They don't need a multisite installation to make multilingual site. They transform a standard installation in a multilingual site.
I believe this plugin allows you to link different (multilingual) sites
http://wordpress.org/extend/plugins/multisite-language-switcher/ (I never tested it)
Polylang has not been tested with WP E-commerce. For WPML, I guess you can just ask them.