To me – and this is perhaps a philosophy not many agree with – the language you use in a story should not be a barrier to the understanding of the story. The reader is not approaching your story with the burning desire to work hard, he’s approaching it for entertainment. A strange or intruding use of language must have enough value-added to pay for the extra trouble you’re giving the reader.
This is a very important thing with me, because – having learned English abroad, primarily in the British variant and in a classroom – the word that comes naturally to me is often the ten dollar word, not the twenty five cent one. So I fight it constantly.
Even in English, not going into invented, foreign or dialect type of expression, the "look at my humongous vocabulary" author tends to put me off, partly because I can see them strain to do what to me comes naturally and I fight. Now, there are some that don’t strain, and whose language does provide enough value-added to me, though the only one I can think of right now off the top of my head is Ray Bradbury.
Beyond being a speaker of English as a second language I – being the gift that keeps on giving – am hearing impaired. This means if you try to reproduce dialect on the page, you’re going to bring me to a screeching halt. There are entire bookshelves of authors of the picturesque/regional variety I can’t read. It would be like reading in an archaic language and working it out word for word. In earlier days – when I was merely a graduate student of English, having lived only one year in the US – a word in a Heinlein book popped me out of the story, because it required I pronounce it aloud to know what it was. The word was "purty."
So, my feeling – mostly – on invented and foreign languages in writing is "don’t." That said, I’ve seen it done very well and even I admit that particularly in sf/f giving your alien/different Earth characters or places a set of names that are linguistically cogent with each other is important. So I’ll go into that next time.
Meanwhile, what are your pet peeves on dialect use? Do you disagree with me? (It’s okay if you do, I left the hand-smacking ruler in Texas.) Are people like me such an outlier that the author shouldn’t worry about them?