I predict that within 20 years or so, everyone on the planet will be able to speak the English language fluently. Most of the world already does, and of all the places I have traveled, there is always English speaking people around. It is only a matter of time before everyone learns it.
Yes, there will be a global linga franca, because several have developed already. It is only a matter of time before one becomes universal. Over time, technology has made the world smaller and smaller. With this is the need for a universal language for communication, regardless of mother tongue. That is what global linga franca will do for the world.
I personally think that it will be a global lingua franca because it is a bridge language or a vehicular language which is not often used today. I personally think that it will e a global lingua franca because have arisen around the globe throughout human history as well as it was used to made to communicate with people.
No, this will never exist globally. There will never be one language that is understood by everyone in the world, even though the English language is taking off pretty good and is known by a whole lot of countries spread out all through the world. Right now I would thing there wont be one though.
A linga franca usually refers to a bridge language between two other languages. Overall, I would say we are more likely to see a universal language rather than a global linga franca to bridge different languages. I believe English has probably come the closest thus far but even it receives competition from Spanish in the United States. I think eventually we will favor one language, but I do not believe this will happen in the immediate future.