What is the economical advantage coming from the ability to speak English? I'm very much interested in the difference between anglophonic and non-anglophonic countries stemming just from the language skills. I'll be grateful for any references regarding the topic.

What is the approximate annual economical gain of a country stemming purely from investments in English language classes?
One of the best ways to study the benefit of speaking English, at least within an English speaking country, is by looking at the wage discrimination literature. Some, but not all, of wage discrimination turns out can be explained away by the ability to speak English. These studies are good to look at because of the controls for other factors. For example, it looks like the ability to speak English was a factor in this study, looking at before and after the impact of the IRCA in the US. The references I give of course are not generalizable to the whole world, because of their focus on the US. So I suggest looking more into the wage literature yourself.