Anyone plunged into another language environment has to try to pick up a different set of ways to express themselves. But this need to adapt happens for adult speakers in our home environment- apart from all the different languages spoken in England, there are so many varieties of English. All languages adapt and change because of contact between people. The more contact, the more pressure to change. That doesn't mean that we immediately take on every style of language we come across.
Language is a badge of identity.
We balance a need to fit in, with a need to remain individual. Every person's language is as unique as their fingerprints. Such books are a snapshot of the language, out of date from the moment they are written. Pulling in the other direction is the need to stretch the language, so that new things can be expressed in different ways. Each new generation learns the habits of the old and moves on.