Some time ago, I had a brief conversation with a colleague (who happens to be Canadian) on languages. He overheard me talking on the phone with an Indian friend in Hindi, combined with English shortcut phrases. Overcome with curiosity, he couldn't help asking me which language I was thinking in, while I was having this bilingual conversation. It was a good question for I couldn't tell.
Language is a carrier of the human consciousness, and like it, it's continually evolving. Divided by geography, our ancestors each evolved their own separate languages. Now due to the amplification of our transportation and communication abilities, the world is shrinking, languages and cultures are coming together.
English is the primary language in many countries, mostly the nations that arose after the dissipation of the British Empire. As it was the last great empire - 'on which the sun never sets', so English is the widest understood tongue today. In India, the favoured language of commerce and education is English. Another recent example is the Soviet Union - nations that now constitute the Commonwealth of Independent States (CIS) still largely support Russian. For example, on my visit to Armenia, I encountered Russian still to be the favoured language of business and government. The current adult generation was educated in Russian medium.
Language is affected by the manner in which it is used. Traditionally, it was used for discourse and expression of feelings, it could become very poetic and beautiful. However, when used for formal workflows - business, science, law, government etc. - it tends to become very fixed, industrial and inhuman. That's when we have to start memorizing book-loads of vocabulary.
Enter the digital age - English is the language of computer science. If you wanted to do computer programming, you have to do so in English. One of the reasons that India enjoys such an advantage in Information Technology. I had the very strange experience of learning BASIC computer language in Spain, in Spanish. That was another multi-lingual experience I didn't fully realise until later.
Language is evolving still, with the Internet it's become a medium as well as a technology itself. The rest can probably be explained by the video below, which has been circulating for some time in my work place, and being shown in an awful lot of presentations.