Perfect translation is going to be hard. For an everyday most common 5,000 words that may be ok but anything outside of that would require the invention of many new words when English likely already has them. They don't really mean anything in English either - for example "quantum mechanics" are just two words, and have to be learned even for English speakers. When coming from another language you may as well use whatever words others are already using. You can see that happening when other language speakers adopt English words.

There are other factors - English speakers readily adopt words from other languages as needed. This makes it a lot easier to expand. Compare with the Académie française: http://en.wikipedia.org/wiki/Acad%C3%A9mie_fran%C3%A7aise

Look at what people all over the world are constantly bombarded with - Hollywood's output is shown (and popular) everywhere. As are words like Coca-Cola, McDonalds, Nike and Apple. Even Bollywood is including increasing amounts of English in its output. Popular entertainers in various countries have also taken to singing some or all of their songs in English.

Look at generating input. There is a bias to a reasonably small number of symbols. A 500 key keyboard isn't practical (although variations exist). Having many modes and switching amongst them isn't that pleasant either. We already see people dropping case from English where is it mostly cosmetic. (Of course voice recognition is always around the corner but isn't practical in many environments and has always been around the corner.)

It is acceptable to speak English "badly" - did I tell you about the time that: the mat cat sat upon? I've heard from speakers of other languages that just getting the gender wrong in a sentence is enough to make it extremely hard to understand.

My prediction is that the number of speakers of English as a second language will continue to climb since it is the most friction free way of communicating in many circumstances, and that will rapidly lead to their kids adopting it as a first language. Heck all the people I know who have a non-English language as their first language have their kids speaking English as their first language.

TLDR: English makes a very good lowest common denominator