Excellent question... and one that I've got both experience in and a definitive answer to. Particularly as an undergraduate, you will only show a mature and abiding interest in the field if you can identify one or two specialized areas of interest that get your intellectual juices flowing. Folks who are at the stage where they, for instance, "really enjoy German history" or "fully appreciate the complexity of Genetic Biology" are undergraduate majors, not prospective graduate students.

This is done for your benefit and for the benefit of the program. You can spend the rest of your life studying Spanish Literature (as you apparently hope to do...!), but in order to get through graduate studies you need to accomplish three tasks. First, pass your general exams, which will be detailed questions to illustrate your understanding of the field, generally focused on three settings (usually defined by a small range of years in one geographic setting). Second, you must fully and completely understanding the intricacies of the one or two facets of the discipline (topical or regional) which will allow you to ground your thesis within the scholarly literature. And the third and penultimate task is the dissertation, in which you must illuminate in scholarly detail a topic that only perhap a dozen folks in the world will be able to discuss with you as a peer. From this dissertation, you will make your name, your academic resume, your conference papers, your scholarly network... and likely your first job.

Many people succeed in passing generals, but the vast majority who enter graduate school fall before the gargantuan task of the dissertation. So if it is the goal of the department to have successful graduate students who will enrich the reputation of their program by completing stellar dissertations and landing glorious jobs in the upper echelon of academic excellence, they will rightfully be concerned about applicants who are unwilling, unable, or simply unprepared to indicate a specialized focus.

Of course, all of the foregoing does not preclude your ability to change your area of interest upon arrival; in fact, I'd estimate that perhaps half of the students do just that. Some change their focus entirely (e.g., from French 18th century history to American 19th century history), but more often subtlely (e.g., from economic investigation of the American Federalists to a religious-political understanding of American revivalist rhetoric). But when you arrive, you will hope and need to find an advisor that fits your current interests well... and whose mind you respect. If you can fulfill these basic tasks towards focusing your interests and identify specifically who can help you achieve your goals, you can indeed succeed in grad school.

I'd recommend that, little by little, you "try on" different focal points for your studies and see how well they interest you. Because when you finally decide on that dissertation topic, you've got 5-7 years ahead wherein your very existence will be surrounded by that mass of information. Whether in conference papers, job offers, and future works developing other areas of the topic, it'll be your baby. If you've at least gone into the process with your eyes wide open -- and tried on a few henna tattoos before you decide to have one permanently affixed -- you'll be much happier and successful in the long run.

Hope this helps and I wish you the best of luck!