It depends on what you're planning on studying. Usually it is best to learn the region you're planning to study as you'll be reading primary texts from archives usually in the current regional language. I plan to go into Napoleonic scholarship or general French history, so I need to know French and it would help if I learned German since German speaking peoples played a part in French history and vice versa (although Latin would be very useful because the Habsburgs wrote laws in Latin).

So, what are you planning to go into?

From my experience working in a Department of History at a major university, I can tell you that it depends largely on your focus. My focus in eighteenth century history is America, in which I've found that there are four primary languages for exploring primary sources: English, French, German, and Spanish.

By and large, English, French, and German cover the bases of cultural development in Europe during the eighteenth century. This is particularly true if you're studying literary, musical, and artistic developments. If there is a particular region of Europe that interests you, focus in on that. Italian, Russian, or Spanish may be more beneficial depending on what you study.