Is it possible to enroll in a graduate program completely unrelated to one's undergraduate degree? Additionally, in my case, my undergraduate degree (computer science) is from outside the USA, and I'm applying to graduate programs (in English Literature) within the USA. Will this be a problem?
You should definitely apply. I have seen many cases where individuals with degrees in tangentially related fields not only are accepted to, but excel in, graduate programs in different fields.
Do note, though, that if you're applying to be a PhD, it's expected that you'll be doing research in the new field, and you'll have to demonstrate to the application committee that you're up to the task. You may have to be creative about this, given that your background is very different from what they're used to seeing; consider emphasizing any possible applications of computer science to literature (ngrams, maybe?) or anything else which may strengthen your application.