English is dominant, not because the number of speakers, but because they are so widespread. Mandarin has more, but they area almost all in China, whereas there is hardly a corner of the earth where you can't find an English speaker. So if you plan to travel to a lot of places, the most efficient language to learn is English. Between any two non-neighboring countries, the language with the highest number of mutual speakers is almost certainly English. I don't think any other language in history has had this level of network effect.