The simple answer is you can't. Keyboards are "dumb" devices and aren't
programmed for one language or another. You physically just change what's on
the keys and then have to tell Windows what keyboard is plugged in. That you
do by using the language bar, but you will have to define English as using
an English keyboard and Spanish a Spanish. Also you should be aware that
some programmes, such as Word, "automatically" detect the language it thinks
you are typing in and will change the language bar accordingly.
By the way, is it really worth the bother? I would just leave the Spanish
keyboard plugged in and work on that. I don't work in Spanish, but do have a
Swiss keyboard, which allows me to work natively in French and German and
there isn't enough difference with an English keyboard to make it worthwhile
physically changing keyboards when I use English (which is most of the
time).