I know there are lots of paradoxes in all this, but in the end we have to keep reminding ourselves that it's just people that speak language not abstractions like a nation or a culture or ethnic group. I began learning Japanese about 15 years ago. When I look at young people on the street, it seems like they're speaking something that I never learned in the classroom. The languages that don't change are those which are fixed into a writing system, like Latin or Greek, and which don't have a speaking population. When you have a speaking population of 120 million people it's normal that it will change again and again.
Language has a kind of dual track.
One track is conservative and the other track is creative and innovative. Language is so conservative that a 90-year old man can walk into a shop today and easily hold a conversation just like he did 70 or 80 years before. On the other hand, people move in and out of different networks and speak with different and rapidly changing styles. Linguists are interested in tracking those changes. Changes in grammar and in vocabulary.