I second your request (even just a simple toggle would help A LOT). In fact, I wanted to post a thread/feature request about it some time ago
. Post it on launchpad, as @BetterRed has already suggested (and share the link here).
@BetterRed, unfortunately this doesn't work even for physical libraries. This setting is set per Calibre installation.
I think using the language code to trigger the right Amazon search wouldn't work for me (when I download metadata, I often don't have any language code or don't have the right one). I would prefer simple checkboxes in the settings (to check the Amazon stores I want to use for the search), and then initiating automatically multiple searches (as in the case of using multiple search plugins).