For someone starting their career today, which foreign language will ity be most useful to master in the future?
Spanish or chinese i’d have to say. Although german is useful too, and has the bonus of making you sound tough when you speak it…
I would actually say that allthough it will be a huge demand for Spanish speakers, we do allready have the necessary supply. It is unlikely tyhat we will face any sort of shortage of Spanish speakers. Would a combination of two languages from the same region, say mandarin/japanese/korean, open up many opportunities?
English. Oh, you mean foreign language for native English speakers. 
Really, the question is too broad. I speak Japanese fluently now, which is extremely useful for living in Japan, but wouldn’t help as much if I really wanted to live in Greece.
If you are talking about business, while learning a foreign language is useful, living in a foreign country shouldn’t be overlooked. There’s nothing like it to really learn the culture and the language.
If you can handle it, sure. I know a lot of people fluent in one but very few who can handle two.
Arabic might be good.
Could you elaborate a litle on that?
Um, Arabic might be good as a candidate for a useful language.
Given China’s predicted economic boom, one could certainly be in demand as a Mandarin-speaking nanotechnologist (from your other career-related thread). You could say you know a little Chinese!
Sorry.
A vote for Mandarin. Sure two asian languages would be great, but that’s like taking 10 years out of your life.
The thing is, if you choose to learn Mandarin (or Japanese), you are pretty much signing yourself up to learn 2 languages, since learning to write either of those languages is almost as hard as learning another one. And before anyone nitpicks me about this, note that I said “almost”. But then again, if you learn to read Mandarin, you have a leg-up on learining to read Japanese… so the equation is actually pretty complicated. 
Although I tend to agree with the earlier poster about Spanish not being in great demand (due to the availability of speakers), if you don’t have a lot of time or optitude, you can probably learn to speak passable Spanish in a lot less time than you can learn passable Mandarin.
I would say Arabic would be useful mainly for a career in government, not in business. Unless things change in the Arabic world sometime soon.
i personally want to learn Spanish, i hear people speaking Spanish around me all the time and i want to know what the **** they’re saying.
In business, many companies package their products in English, Spanish, and French. Due to colonialism, these three languages have evolved into lingua franca around the world. Fluency in the three would surely give you a major boost in your career.
I would put in a vote for Portuguese, especially if you will be dealing with South America.
I’m not convinced. Well…actually, I’m convinced it’s great to understand english, french and spanish as communication tools. But the OP is worrying about his career, not about gaining the ability to communicate with as much people as possible.
And the fact that these languages are so common precisely means that there won’t be any shortage of speakers. Possibly an useful skill, but not something which will result in potential employers looking at you with awe.
I would tend to think that all languages can be equally useful. If you learn english (assuming you’re not an english speaker at the first place), it will be very often useful/required, but plenty of other speakers will be available. If you learn Romanian, you won’t find many jobs requiring it, but on the other hand, you won’t have many competitors for these jobs.
In fact, I would say it highly depends on what career exactly the OP envision. One probably won’t need the same language skills in the oil industry and in the diplomatic field.
Actually, I would tend to think that the OP should choose the language he has the most interest in. It will make much easier and pleasant to learn a language you like at the first place. Also, using professionnally this language will probably mean that the OP will have contacts with people from the corresponding country, will travel there, etc…So, once again, it would be much more pleasant if he had some interest in this country/culture.
Also, if the only goal of the OP is to add something to his resume, I’m not sure a foreign language is the best choice. Learning one requires a lot of time and investment, so one (especially an english speaker) should do so, IMO, only if he already has a real interest in languages for their own sake, or is pretty certain that he will use this skil professionnally. Learning it “just in case”, without real, serious prospect of ever using it would be a waste of time, as he could get instead a degree in some field related to his future job with an equivalent (or perhaps less important) investment.
Now, learning Mandarin, as proposed by other posters could be great, but it’s not like the OP will be the only person thinking that in might be a valuable skill in the future.
It made me think about the signature of a Brazilian poster on another board : “Brazil is a country with a great future lying ahead and will stay so” (or something similar).
No, seriously, I do believe Brazil has indeed a great potential and will become a heavy weight within a generation.