When people talk to each other, assumptions are made about their common knowledge. So depending on who you are and who you are talking to you are going to use different words in different ways. It might be the case that the colloquial form of a language actually reveals a lot about the culture that gave birth to it that the written form doesn't. Maybe the writing makes sense on a superficial level, (the colour green, etc) but actually has a lot of subtext that only becomes clear when someone from the same background as the writer reads it. You can get away with deducing a lot of this from other pieces of writing, but what about culture that don't heavily rely on writing for communication? Or learning about parts of society that didn't write much?