I was thinking about this after I went to see it a second time.
The actual fact that they find out humans aren't alone in the universe isn't really a big issue in this film. The main issue is 'where do we put these 'prawns'? and also how they can take advantage of their advanced weaponry.
Not 'wow!!!! Aliens!!!! We're not alone!!!!! No one has really asked these aliens where they came from or why they're here or maybe they have and I wasn't listening properly.
Surely in real life, if this indeed did happen, there would be millions of people flocking to Johannesburg from every country in the world, just to see these aliens, maybe even creating a booming tourist industry for South Africa. I know I'd want to go see them with my own eyes.
I would also expect the major powers of the world (US, EU, Russia) and other organisations to take control of the 'prawns' or at least help S Africa out with their problem.
Oh well, mustn't ask too many questions.