major in whatever you wish, but there is one thing i would be careful about, the parts of your major that deal with language, like 'spanish language 101' etc., i'd find a way to bypass those language classes which you know the material, maybe get credit for them already etc.
One of my friends who was korean tried to take intro to korean language, and to start the class, the professor told a joke in korean, and anyone who understood the joke and laughed a little, was kicked out of the course immediately. I just want to say, be careful what your doing, if you truly want to learn the history and parts of the spanish culture you don't know, then do it. But, if the plan is to take spanish language courses, which you already have full knowledge of, imagine if you are caught. IN a profession where ethics is valued at the top, is this really the way you want to begin your career?
Anyway the choice is yours.
EDIT: For example many universities have you substitute the basic language courses for a different type of course is you are a 'Heritage Speaker'. "* Heritage Speakers of Spanish must substitute other courses for these. Contact an advisor." So yes, its very possible for you to major in spanish, rules are already in place for students as yourself.