Many people think the term arose because of the dominance of French in diplomatic circles, but I t's actually quite apropos; "lingua franca" is an early medieval term that essentially translates to "language of the Western Europeans" ("Franca" in this context is not modern France, but a term for historical Germany -- everything north of modern Spain, essentially). In the day, lingua franca was a patois consisting of bits of various languages that all traders in the region knew and could communicate in, though it was too mutable to be a stable language as such. Not unlike English, actually!

It helps that English adapts to the current zeitgeist relatively fast and can kind of act as an ombudsman for the spreading of news concepts and ideas to other languages.

True, it does seem like a contradictory sentence. Here, I'll fix it: "English is the common language of the world." :)