What determines how language in a particular country evolves? Why is French for example, so different from German? 

There's no simple measure of how different languages are from one another. In fact, if you look at them the languages, structurally, the way a linguist would look at the French is different from the other Romance languages in a variety of ways, which make it more similar to the German and other Germanic languages. There are number of features of French which are sort of Germanic and character, incidentally, Old French, middle France, a French in the medieval period was not it was like the other Romance languages. So something happened to it that made it less like the Romance languages and more like the Germanic languages. 

How does language change over time? 

How did 18th century French change compared to 12th century French? 

But when we talk about language change, that's very misleading. I mean, up until the turn of the century, you could find people in nearby villages in France with virtually could not understand one another. The idea of a national language is a pretty modern phenomenon. It has to do with the rise of nationalism, and communication, and so on. And when we talk about language changing, what's is actually happening is that it's kind of like species changing. There's a mixture of all sorts of dialects, and the mixture of these things changes over time. And you take a look at it a few centuries apart, it looks like there's a different language. I mean, within a couple of generations, the language can change structurally in quite dramatic ways and of course it's a lexicon, the words of the language, well, that's a different matter altogether. 

So when technology develops, you get a whole new vocabulary. But if you were in France in the 12th century, and you understood all nuances of language, could you have predicted how these various languages would have evolved over time is a partially random? 

It's not actually random. For all we know, it might be completely deterministic. There's just to many factors involved. Speakers of English can be misled by this. English is relatively homogeneous. I mean, I just came form Boston and I understand everybody in Portland, but that's not true most of the world.