I used to want very badly to become fluent in several languages. After living in Europe, however, I finally came to the realization that the effort involved would only be worth it if I was doing it for the sheer joy of learning.
There was really no need to learn any language beyond English. The whole world was learning English and it is even more true today as people use the internet, get American tv and radio (increasinly over the net), and so on.
As English incorporates other vocabulary from other languages at an increasing pace, this will make it even more interesting and dynamic. But the effort to learn, for example, German would just be for art's sake; there's really no practical value as most Germans speak English.
Arabic might be very handy to learn, at the very least to translate the imams' incitement to violence sermonized at mosques.
"After living in Europe, however, I finally came to the realization that the effort involved would only be worth it if I was doing it for the sheer joy of learning."
Like you, I lived in Europe (Germany). I arrived just four years after taking three years of high school German.
Unlike your realization, I found that one had a different relationship with people, when you spoke with them in their language, regardless of them also speaking English.
Later I worked in international business for several years, and with professionals from around the world. Again there was reinforcement of my earlier experience.
It is certain that English has become the universal world language, much to the dismay of the proud French. When I studied, German was for math/science and French was for diplomacy.
Now it is English across the board. It is used by ALL airlines the world over, on international flights (so I recall hearing).
Yet I still maintain you can develop a closer relationship with someone if you sincerely demonstrate an interest in learning their language. It is almost always reciprocal--which I think is the point.
Having said that, not everyone holds "good will" in their hearts and basic cultural characters. We know the terrorists are often well educated, willing to learn English so thy can deceive us, then kill us.