But I have studied several foreign languages both in school and on my own. So, for what it's worth ...
First, ask yourself why you want to study a foreign language. Is it for personal edification? Is it for a specific job (Chinese because I'll be working for a Chinese company), a specific academic program (Arabic because I'll be studying Islamic literature in grad school), or a specific circumstance (I'll be marrying a Russian girl and I think I want to learn Russian)? Is it to find a job in areas that require a foreign language (I want to teach X to high school students)?
If it's for personal edification as in I want to learn X because I like it, then, by all means, learn X for no other reasons than that it pleases you and don't look back.
If it's for a specific job, academic program, or circumstance which you have planned for the foreseeable future, then you don't have much of a choice. Whatever language is required of you, you learn it.
But if you don't really have any specific short-term or long-term goal, and you want to invest your time, money, and energy learning a foreign language in preparation for whatever opportunities that may come your way in the future, here's what I think.
1. You should go for languages that are spoken by multiple ethnic groups living in multiple areas of the world. You get the most bang for your buck that way. In light of this, it's not hard to see that Japanese, Thai, or German (http://en.wikipedia.org/wiki/File:Map_German_World.png) will not create nearly as many career opportunities for you as, say, Arabic (http://en.wikipedia.org/wiki/File:Dispersión_lengua_árabe.png), French, or Mandarin (http://en.wikipedia.org/wiki/File:Mandarin_and_Jin_in_China.png) plus Chinese diaspora).
2. You should go for a language that shares the same family with other widely-spoken languages. Family members behave alike. Once you know one in a family, you pretty much know how to deal with the other members of the same family even though they all have certain features and idiosyncrasies that are unique to them.
Take, for example, Romance languages. Two major, major languages in that family alone: Spanish and French. (Italian, Portuguese, etc., are great languages to learn, I know, but see point #1 for the reason I've singled out French and Spanish.) This means, once you know French, adding Spanish to your repertoire at a later time is a piece of cake, and vice versa. Knowing French will serve you well in these areas: http://en.wikipedia.org/wiki/File:New-Map-Francophone_World.PNG. Knowing Spanish will get you places in these areas: http://en.wikipedia.org/wiki/File:Castellano-Español.png. With either French or Spanish under your belt, if you have the time and the desire to study, say, Catalan, later, guess what? Easy.
This applies even to dead or ancient languages. Members of the Semitic family is well-known for behaving alike, very much alike. Pick one of them to study first, and you'll find that acquiring competency in the rest is a cake walk. Going from classical Arabic to classical Hebrew or from Latin to Attic or Koine Greek is easy, much easier than going from ancient Khmer to Sumerian.
So the whole biggest-bang-for-your-buck thing from point #1 also applies here: go for a large family with multiple influential members.
The question you may have at this point is: wouldn't studying a language that most people study make it hard for me to stand out, find a job, be considered unique and indispensable?
Yes and no. Some languages are more popular than others for a reason (which will be elaborated in point #3 below), and some languages are ignored for a reason also. True, there are a whole bunch of people who know Spanish, French, and Arabic, but how many of them know Hmong? What if I set out to become an expert in Hmong? Or modern Aramaic? Or Estonian? Or Ainu?
You can. But you're gambling. You could win big. You could also lose big. How to decide? Consider the worst case scenario, if you're okay with that, I'd say go for an obscure language or a language that may not be obscure but doesn't really have that much impact on the world compared to others (e.g. Catalan, German, modern Greek, Polish). But even with an obscure or a not-so-obscure-but-not-so-in-demand-either language, you probably also want to keep the following point in mind.
3. If a lucrative, financially-rewarding career (well, as rewarding as a career in the humanities is, anyway, which means not very rewarding) is a goal, then consider world events. Back in the Cold War era or even back in the early 90s after the disintegration of the Soviet Union, knowing Russian would definitely land you some really good jobs both in the public and private sectors. In the last decade or so, Arabic has become one of the top, if the top, choice among those seeking to learn a foreign language. Due to the war in Afghanistan, even the Pashto language (which is not nearly as widely-spoken as Arabic) has paved the way to many governmental jobs with ridiculously high salary. With the rise of China, Mandarin Chinese has also become an extremely popular choice.
Knowing a language that is in demand will naturally open up more opportunities for you than knowing an obscure language or a language spoken in areas of the world that aren't the current foci of the world's attention.
Now, once you've chosen a language that makes the most sense to your situation to study, what do you do with it? What jobs can you get with it?
Most people who get into foreign languages often harbor the dream of traveling extensively or living overseas. From my experience, though that's a real possibility, realistically, you're not going to be making much money at all. You could spend several years living frugally overseas only to find yourself back in your home country with no retirement funds. This has happened to people, mostly those who work for nonprofit religious organizations, whom I know personally.
There are people who live as expats overseas who make tons of money and can afford to live in apartments and homes priced in the range affordable only among the local elite. But 99.99% of the time -- fine, based on my experience, 100% of the time -- these people's jobs have nothing to do with teaching a foreign language or serving as a translator/interpreter.
Most likely, you'll end up teaching English or doing something for non-profit organization (read: not much pay).
Work as a translator/interpreter is best done as a (hopefully) paid hobby or a side job. Translation gigs are hard to come by, and when they do come by, they come by with very little money. In fact, when it comes an event or a task wherein translation of X into english and vice versa is required, most companies or hiring entities much prefer a native speaker of X who also knows English as a second language over a non-native speaker of X whose first language is English (assuming all of this takes place in a country where X is the official language).
But there's always the Foreign Service.
Quite honestly, if the goal is to travel and live overseas, it's best to make TESOL your main study and a foreign language a secondary endeavor. TESOL will land you a job overseas (teaching English, running an international school, etc.) much more easily than a major in a foreign language. Just my two cents.
