This really depends on what exactly you are interested in as a historian. Even in medieval Spanish history there are a lot of areas of study where Arabic is not necessary because all your primary sources are in Latin and maybe in some Romance vernacular. As to reading modern historiography your working languages would probably be Spanish (of course), French and English. To give you an example from my personal perspective as a medievalist, my primary sources are in Latin, medieval Catalan and Occitan and most of the historiography is in Spanish, French, English and Catalan with an occasional article in Italian. I have never felt I needed to learn German, for example, as the most recent study in that language I ever came across in my work was published in 1918 or so. On the other hand, were I to pursue a different field in Spain I would have to learn medieval Castilian, Arabic or maybe even Hebrew or Greek.

In the field of History, there are no explicit language requirements beyond the ordinary language classes which must be passed in order to fulfill the language requirement of whatever university an individual graduates from. That being said, certain language knowledge does help with whatever a given historian wants to specialize in: if an aspiring historian plans to specialize in the history of early medieval Spain, working knowledge of medieval arabic can indeed be very useful. On the other hand, a historian interested in the reign of Charles V (or anything else in the early modern period, or Spain after the War of Spanish Succession, or whatever, you get the idea) would probably be better served studying French, or whatever language is most applicable to that specific era. As another example, a historian interested in the Roman period might want to familiarize themselves in Latin.

There is no precise "formula" required to be a historian anywhere, even in Europe (where licenses and examinations are a fact of life). An aspiring historian is merely required to learn whatever they need to know in order to subsequently research and publish works that hold up to peer review.

If we'd like, there's a whole discussion to be had about the specific relationship that Spanish historiography has with Arabic sources ("Spain was never invaded by arabs," is a proclamation infused in a lot of Francoist historical works). And there's an interesting dynamic whereby there could probably be more frequent cross-referencing between historians studying medieval Spain from a primarily Arabic perspective and historians coming from an Iberian background, but I don't think that's what your asking about. So in sum, the answer is "Yes, a Spanish medievalist would probably be well served by learning medieval arabic, but it is by no means an explicit requirement. A historian specializing in other aspects of Spanish history would probably be served by learning some other language."

Also, it is expected, but not a requirement, that you should be able to read modern languages that could help in your research. English is the one that people tend to assume or at least recommend. A lot of literature is published and most of the most prestigious journals only accept papers in English. Therefore, it is highly recommended. Then you could include others languages. For cultural reasons, a lot of people, at least in Catalonia (a province of Spain) could learn French. Moreover, in medieval history, the French historians have a very significant prestige. On the other hand, if you study something related to Rome, Italian could be a better choice. But,in the end, this is no requirement. Some people just limit themselves to Spanish and they face little or no problems for that.