Internal Server Error after changing settings

After installing the plugin, I was adding the custom content for the second language. When I switched to changing the general settings and saved, I got this error. Now I can’t login to change anything from the backend. It also looks like the url has already the language string added but such a url doesn’t phisically exist.
I try to go here: http://inn-realestate.com/wp-login.php and I get this: http://inn-realestate.com/es/wp-login.php which doesn’t really exists.

How could I fix this?
Thanks.

http://wordpress.org/extend/plugins/stella-free/