Wikipedia has this already if you have an account, and dig though the beta features, but in this is a good implementation for those who do not want to sign in

I really like this Addon. But I'd like to have the function to open this kind of popup-window after selecting text independent of the Wikipedia site, searching for it and present the successful result in this small 'window'. And the option of preferring English or other language searches. It would be really nice. Then I would rate for 5 stars :)