In 50 years, the majority of the world will speak English, Mandarin (Chinese) or Spanish. The Chinese are teaching English to their youngest generation in a big way. So Spanish will be the most "useful" language to learn.

Spanish is also easier to learn than Chinese. It is pronounced the way it is spelled, and uses the same alphabet as English. Fluency in Spanish will greatly increase your enjoyment as a tourist in Spain, the Canary Islands, most of South America, and much of the Carribean.

Of course, if "usefulness" and "ease of learning" are not your criteria, the field is wide open. French and Italian are beautiful languages, and open up the enjoyment of French and Italian movies. I would not bother with German. It's not an attractive language to my ear, and young Germans speak great English.