Lingua Franca

There are so many languages in use around the world today, most commonly demarcated geographically by political borders. When the Eurostar spits you out on the French side of the Channel Tunnel, you are no longer guaranteed to get by with the lingua franca - English. What is the force that drives such a sharp and instantly recognizable divide? Is it a cultural thing, a human thing, that we wouldn't ordinarily regularly associate with other people from so far away? Add to that the state media of television and newspapers in the local tongue, where we in the first decades of the 21st century probably get the highest amount of indirect human influence as far as language is concerned. I like to believe in a past where languages spread through the world like oil paints on water. Then, as geopolitical boundaries - such as countries - appeared and solidified, the dominant colors filled those shapes, giving us the simplified view we've grown to expect today: when in France, speak French.