The site's world has indigenous languages aside of Common, so is it all right if one actually uses them? For example, maybe someone would like to study Old Serenian or to speak a little with someone as part of a thread's story, but perhaps this someone will come up with certain words or grammatical features that another person might not want, and this way things would not match.
So, the question is, are people allowed to invent, at least to an extent, the structure of the languages?