The way that languages develop is a delicate balance between two powerful tendencies. Pushing in one direction is the need to conform with existing conventions. The most obvious is the way infants absorb the language they hear and experiment with those sounds to find ways to communicate. 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.