Hey, if you're conducting business internationally - never mind just in English speaking countries - you're probably doing it in English. English is the current Lingua Franca of the World. All First World countries are teaching English to their children. Upwardly mobile Second World countries, such as China, are trending towards English also. Why the U.S. should be trending away from English is beyond me.
1. English--this is the language of business and science, first made important by the British Empire during the 17th to mid-20th Centuries and since then by the Americans.
2. Spanish--this is the language of the vast majority of people living south of the USA. Even the Brazilians usually have to learn Spanish to deal with the rest of Latin America.
3. French--besides being very useful in France (the French are somewhat adverse to speaking foreign languages), French is very useful in much of the African continent because French is likely the ONLY common language among many Africans who speak a myriad of local languages and vernaculars.
4. Mandarin-dialect Chinese--given the size of China (1.4 billion citizens) and the fact the Chinese government has encouraged everyone in the country to learn what is in effect the Beijing-area dialect of Chinese, knowledge of this language will be useful as China emerges as a major economic power.