Where are you located?? Where do you want to fly? What cultures interest you?

I started out learning Spanish in high school because I lived in an area with lots of Mexican immigrants. I later realized how much I liked it, how much I liked the different cultures, and became a spanish language major in university. I also started taking French for fun in college and it became my second major. Then I learned Portuguese for fun, Italian for fun, and then I married a Spanish girl from Barcelona and I learned Catalan. (I took German too but I can't really speak much except for simple conversation).

Well, even though English is the international language of aviation, I had found Spanish to be extremely useful. First of all, while living in the US, I had plenty of opportunity to use it. Now that I live in Spain, obviously it is quite useful, but additionally ATC is done in English and Spanish. You will find that Spanish is used alongside English across most of the territory south of Texas. Sure you can get by with English, but knowing that second language helps improve situational awareness. I even teach in Spanish at a flying school in Spain, and airlines here (and in Latin America) are asking for Spanish aeronautical language proficiency.

One of the reasons why I got my current job is because I speak the local language fluently AND I'm a native English speaker.

French also comes in useful if you are planning on doing any flying into Canada. I'm not saying that you need to speak French over the radio, but at least you have an idea of what the other guy is doing.

The three most spoken languages in the world are Mandarin, English, and Spanish.

Someone previously mentioned German. I wish I was fluent in German. Now that I'm living and working in Europe, German could really open up some opportunities. The airlines in Germany require fluent German language ability (in addition to legal right to work). The thing about German is that it is not used in ATC except for GA for the most part.

By the way, someone mentioned that German may be easier to learn because it is in the same language family. This is true. Similarly, after I picked up spanish, I picked many other romance languages since they are related.

In the end, just pick a language that most interests you, whether it be because the way it sounds, the associated cultures, or whatever. It will motivate you more.

Some languages you may want to consider because they cover a large geographical area and diaspora are:
Spanish
French
Russian
Mandarin
Arabic