which foreign languages are in most demand around the world? I want to learn those languages to enhance my career.

Firstly, what is your career? Certainly Spanish will cover most of latin America and Brazil's close portugeuse. Of course, you may meet up with a rainforest indian or, in my case a family of Mixtecs picking california spinach who knew less spanish than english! Chinese and japanese are pushed for business in the far east. Inversely, lots of chinese and japanese are learning english. It really hinges on your career. If your a Mozart loving musician definetly german.

If you're in America, and unless your career directs otherwise, learn Spanish. You already know English. Next to English, Spanish is the language that will be most spoken in the U.S. in the coming decades.

China will only continue to grow in economic importance, so Mandarin Chinese is a great choice.

But it also depends on what sort of career you're contemplating. Languages popular for a business person is not the same as the languages most in demand for the diplomatic corps, for example.

If you're in the white collar corporate world, Chinese, specifically Mandarin Chinese.

in my experince if you can speak English, Spanish, and French you should be able to communicate with no problems pretty much anywhere. Chinese is a good choice too. I doubt many westerners speak chinese fluently so that, in itself, would be a highly valuable skill.

I tried to learn Chinese once, it was a lost cause since I can't carry a tune too well.
Chinese is very easy in terms of grammar, but very hard in terms of pronunciation (its a tonal language) and also having to learn a lot of characters for reading and writing doesn't help.

Learn Spanish, French and possibly Portuguese.

Chinese is already very important, Chinese is starting to compete with English as the lingua franca of business. Spanish is tremendously important in the US and Americas. One language I haven't seen mentioned so far though is Arabic. A fifth of the worlds population is Muslim and while not all of them speak Arabic or even the same dialect of Arabic, Arabic is a tremendously important language for business in the Middle East, North Africa and parts of India and Pakistan. Learning classical arabic would serve you well. On the other hand, if you're doing business in India your choices swing towards Hindi, Urdu or Gujarati. Of course the problem with India is that there are so many different languages spoken.

It all depends on where you're doing business and who you're doing business with.

Most of the worlds business is done in English. Think about it, to do business with Americans and Brits you need to speak English. Spanish is the second most spoken language here in the U.S. English is pretty much the most desirable language in the civalized world right now.

Most of the world's business is done in English, but he already speaks English. It'd be better for him to evaluate who he'll be working with and learn their language. Spainish won't do him a lick of good if he's working with Moroccans or Saudi's or Hong Kong businessmen. Spainish is a great skill if he owns a small business but if he works for a company doing international work its only the best choice if he's working in South or Central America.