It depends on which area you want to focus on. English is an obvious must and Latin is pretty much a requirement if you want to go beyond just a degree in History (at the University I go to knowledge of one ancient language is a requirement for an Ancient History Honours degree). If you want to focus on classical languages (particularly Latin), German is important, but it is absolutely not necessary. With Medieval history I would imagine it depends on what region you want to focus on - if, for example, you want to study French history, then learn French instead of German. Really though, English and Latin would get you through without a problem.