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?
An excellent question!
I believe some of the mods have played around and worked with languages but it's currently been put on hold as we work to bring you some new exciting lore.
As a rule we tend to just use a different font, italics or even bold font to indicate that something is being spoken in a different language in threads. We really try to keep everything in English for the sake of other players. You are free to invent your own languages and dialects though. For the canon ones I believe for now let's say no. The admin are free to overrule me on this though.
Hope this helps!