
Even most people that DON'T know Spanish still know that "Español" means "Spanish", not "English". My best guess is that Google, attempting to make a more comprehensive translator based not only on the words used but also the meaning behind it, decided that "Español" is supposed to signify the current language that you are speaking in, and thus the translated version comes out as "English" instead of "Spanish." That still doesn't make it a very good translator, though.
I like how it translates "debes que aprender Ingles" as "you must learn English", "Debes que aprender Ingles" as "you need to learn English", and "DEBES QUE aprender Ingles" as "THAT MUST learn English"