How do people from different places understand each other?

You're in Tokyo and you need the bathroom. The sign says お手洗い. You're in Paris and you want bread. The baker says "pain." You're in Cairo and someone greets you with "السلام عليكم." How does anyone figure out what anyone else is saying?
Here's the thing: there are about 7,000 languages on Earth. Each one is a complete system — its own sounds, its own grammar, its own way of chopping up the world into words. "Blue" and "green" are one word in Vietnamese. Russian splits "blue" into two different colors. Korean has seven different words for "rice" depending on whether it's in the field, in your bowl, or burnt at the bottom of the pot.
