If it weren't for writers using their everyday language and translating things into that everyday language despite the shock/horror of how 'uncouth' it is, we wouldn't have Beowulf, Chaucer, Shakespeare or the Bible - the latter being the root of how the entire world has been shaped over the last 2000 years. Or a functioning legal system.
It's an immensely political and sociological matter to have a groups' own words represented, legitimised and shared. It makes ideas accessible without relying upon somebody else (such as a priest) to interpret them. It enables people with less power to check for themselves if the Law actually says what they are being told what it is. It enables them to communicate ideas with others. It enables them to trade items, ideas and is independent of an education system that may be inaccessible, extremely controlling/politicised, acting directly against the interests of the subjects or being used as a tool of oppression.
That's why historically, oppressed populations have their languages erased - to remove their power, cohesion, identity and communication.
Populations having the power and freedom of their own language is a very dangerous thing for some.