Spanish, French, German are three languages that are good to know since a large part of the world speaks them. Spanish and French-speaking countries make up most of Africa and South America, and while German isn't as widely spoken worldwide, a lot of European countries speak some German and/or are related to German, so it helps to know some of it.
As a Swede, learning German was fairly easy since it's related and a lot of words are similar. Goes both ways, if you speak German you might be able to catch the rough meaning of some Swedish sentences and be able to have some sort of half-understanding of certain words and phrases.
Further, knowing Spanish, French and/or German helps when learning other languages. As I said, learning Scandinavian languages is helped by knowing German. Learning pretty much any latin language is helped by knowing Spanish or French (I learned a bit of Italian and I can half-understand some Spanish and Portuguese because of it, for example).