I have to be honest, this is an area of Worldbuilding that I don't have the patience for. I mean that on a personal level; I don't have the patience to do it. I think it's really cool when done well. But that's not going to be me, unfortunately.
I say unfortunately, because on some level, I wish I did have that kind of patience. I think, when it is done correctly, it adds an incredible amount of flavor and character to one's world. I have some sense of linguistics and the rules one should follow to make a new language. My main character in Crown of Druthal (now trunked) was a linguist. And, after all, I am married to a polyglot. But even though I can totally get lost in mapmaking or history writing, trying to crack the spine of a new language can't keep my attention.
I do try and at least work out the broad brushstrokes of a culture's language, though. If they use alphabetic characters or pictographs. How sentences are structured. Words that represent unique or key cultural ideas.
(And I remain a big believer that one should never make up words when a perfectly good word already exists.)
I do like coming up with cultural quirks that are expressed through language and grammar. I came up with all sorts of craziness for the Poasian language. Verb conjugation diesn't just have first, second and third person, singular and plural. Second person is split into three different categories, dependent upon the relative social rank of the speaker and the person being spoken to. Third person is also split the same way, with an additional split depending on if the person is present or not. (Which creates a marvelous way for a high-ranked Poasian to dress down an underling-- instead of speaking to him using second-person-inferior, they could use third-person-inferior-absent. In essence saying, "I think so little of you I will pretend you aren't even in the room." just by using a different conjugation of the verb.)
But the actual nuts and bolts of vocabulary? Can't do it. I have tons of respect for those who can do it.
Anyone out there know any good constructed languages work? I'd love to check it out.