Out-of-the-box, the default language for the WP E-Commerce plugin is English. As of this writing, the plugin lacks a convenient way for the customer to switch between languages, though hopefully, this is an upcoming feature. However, you may choose an alternate language for display on your store. There are currently nineteen language files available, and whichever one you select will be the one displayed on all of your stored pages. As there is currently no easy way for customers to switch between languages, choose whichever language your customers will expect.

WPML are working on a plugin that integrates with WP E-Commerce and thus gives it multilanguage facilities. You can read more about it here.