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As has been said, it is easiest to adapt an existing language. There are many synthetic languages that you can use, depending on your need and audience. If you were writing a historical romance, fo...
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#3: Attribution notice added
Source: https://writers.stackexchange.com/a/4385 License name: CC BY-SA 3.0 License URL: https://creativecommons.org/licenses/by-sa/3.0/
#2: Initial revision
As has been said, it is easiest to adapt an existing language. There are many synthetic languages that you can use, depending on your need and audience. If you were writing a historical romance, for instance, use of the Klingon language from Star Trek would most likely be unknown to your readers, so it would seem to be something new. On the other hand, it would also lack many of the 'romantic' ideals you'd probably need. Like colloquial speech, be careful to avoid tripping up your readers. If you write in such a deep Southern slang that the reader has to stop reading your story, read aloud the words, listen to what is being said, JUST to understand what the character is saying, then you've probably lost a reader. I might do that a few times, mostly with character names, but after a few times stopping to figure out a foreign word, I'll put the book down.