1 This depends on many factors. In the case of English, Britain was a dominant power, then America continued. It was spread through the Empire, and this continued with capitalism.
2 It depends where you are talking about. In the case of China, there is a boom, and no boom is sustainable over a long period. Do all 600 million learning English really need it? Probably not. However, this pattern is not seen all over the world; different countries have different needs. In India, for instance, the rise of outsourcing, especially over the phone, has led to a great need to deal with pronunciation andadapt to international patterns- English for call centres. Things will rise and fall, but the basic need for international communication will continue.
5 English is the dominant language of the internet, so it wili in all probability reinforce the position. However, this will affect the language greatly- as it becomes more international, it will change and adapt.