It completely depends on what you want to work with for your language requirements. If you want to do American history, then you usually won't have to know as many languages (1 or maybe 2 depending on your area of interest) as compared to say Ancient History (which would be Latin, Greek, French, and German just to start). Spanish won't really be that helpful unless you are working on something about Spain itself, Latin America or something more 20th century American.
All of that being said, don't choose something based on language requirements. As Crater21 said above, you have to love what you are doing first and foremost and then go from there. You could use the language idea to tweak what you want to do within a given field, but definitely don't choose something just based on languages. So for example, if you are really into, say, Jewish immigration to the US at the turn of the 20th century and you would need Yiddish, but don't have that and know Italian really well you could instead focus on Italian immigration in the same period, while building the language skills you need for Yiddish and maybe that will get you interested in comparative accounts. If however you know Italian, but really love Soviet history without any knowledge of Russian, don't just pick something Italian for the sake of it. I hope those examples helped a bit even if they were completely made up...