It's a great addon. I don't know if this is possible but I would like the ability for this to work in Reader View as well (actually that is pretty essential to me!).

edit:
Actually I would also like to see support for different translators. There is an application on Windows called "QTranslate" where you can switch between different online translators (like i.e. Google Translate/Bing Translate). That way you can cover more words.