If the most important languages are the most popular ones, then it is a five-way race between Mandarin, English, Spanish, Hindi, and Arabic. http://en.wikipedia.org/wiki/List_of_languages_by_number_of_native_speakers Note that while Mandarin has a large lead in native speakers, when we look at TOTAL speakers, it's a "two horse race" between Mandarin and English: http://en.wikipedia.org/wiki/List_of_languages_by_total_number_of_speakers And, if we look at the number of countries where each language is spoken, English comes out on top: http://en.wikipedia.org/wiki/List_of_most_widely_spoken_languages_%28by_number_of_countries%29 Lastly, English is the most frequently taught foreign language in the world. All of that suggests a trend: English is evolving into a universal worldwide language. Of course, I could be wrong -- English had once moved towards a worldwide standard with the emergence of the British Empire, which fortunately stalled. And French had once been an emerging world standard too. However, the world is different today -- we are becoming one world highly connected via the Internet with powerful multinational corporations and even nascent world governments. All of that creates compelling reasons and forces towards standardizing. We will be communicating more and more with shared languages. Also, we will likely find it increasingly inconvenient to use unpopular systems such as the US standard measure system (feet, miles, pounds, gallons, Fahrenheit, etc.) I feel very fortunate that English is my native tongue. However, I wish a better language were an emerging standard. English is so complex, requiring us to spend our time debating the meanings of verbs modified by prepositions and American school children to compete in "spelling bees", which make little sense in other languages.

English because this is my own language and is the most useful to me because I like in an English country but I'm sure if I lived in France French would be the most important language to me, but if I grew up with two languages for example Spanish and Italian it would be which ever one I use more.