I know french option
I've just discovered Duolingo with a couple of friends thanks to Techcrunch.com
We really enjoy the exercice, it's intuitive and well concepted. We are learning english and spanish in the framework of our studies but i personaly use Duolingo to learn Portugese.
For the moment there is no option such as "I want to learn spanish and i know French". If you want some help to develop that kind of exercice let me know, we are ok to give you some help if you need it :)
Thanxs for that great tool