There is a bigger picture to it.
If someone moves to another country, then you want to be fluent in the dominant language. Usually most would not have the mindset of having others cater for them. This is common sense speaking at best, but it is only looking at the book's cover.
Part of the reason to why there is no utilization in the fluency unification of the english language may have to do with the fact that due to immigration and nationalization issues, legal forms and written contracts are catered in a matter that it's available in a non-native's language, like say Spanish on DMV tests or tax forms for example. Thus, applicants in this example would not see a need for understanding and acquiring english literacy.
Unless we all can communicate at ease together, we are not truly bound as one unified country. Unification of language should be mandatory to maintain cultural hegemony as unification would represent a host of underlying values, goals, socially constructed ideas. Without an universal form of expression, you can't really become a fully contributing member in that society, only isolated to small minority communities.
The use of language then would be the cohesion that brings this together, it seems more of a minor factor to the overall contribution to unification.
Look at math per example, in it's own world. If someone walked into class and insisted writing everything in base13 or another started using roman numerals.. what do you think would happen?
It's easy to see the principles of mathematics working universally behind worldwide languages - I don't think there is any country out there that doesn't use the standard numeral system and basic arithmetic. As a result, math can be used universally within languages across the world today. The same has to do with facial expressions, such as smiling, laughing or crying, and so on.
To cliff this up one, the true concept isn't about everyone doing a stop drop and roll - "learn english nowz!" - but actually that we need to find a means that we can communicate easily with one another. If the majority language was sign language in america, this would hold true to non-native and non-signers as well.