I personally think almost all fantasy that attempts it comes off as contrived. Tolkien is the exception mostly because of the depth of his expertise mixed with an unusual dedictation that was borne out of his deep love for mythology and country.In spite of them being contrived, I am pretty forgiving about it if it isn't way overdone. I think where people go wrong is when they try to make languages sound exceptionally foreign. But, then, if it's supposed to be very ancient, it should sound pretty doggone foreign.
Robert Jordan's cognates drive me nuts, so I would recommend using real foreign languages to come up with cognates. That isn't to say you can't grap a root here and a suffix there. But mess 'em up good, because I know for sure the people from two rivers were no where near Spain or Mexico, so they shouldn't be wearing "algode," which is an obvious cognate of Spanish "algodon."
In my own work, I simply don't use any non-English terms unless I absolutely have to. And, yes, I think they all sound a bit contrived. If I ever get a PhD in linguistics and ancient languages, I might try incorporating more.
I don't mean to discourage, though. I think creating a whole language would be a lot of fun. And if you find it satisfying, put it in your story. Most will probably be forgiving about it if the story is good--and maybe you'll "wow" us. ![](http://www.hatrack.com/forums/writers/forum/smile.gif)
Anyway, Because I enjoy the subject, I've rambled here for five paragraphs without giving you the slightest hint of assistance toward what you requested.
That probably means you are much further ahead in the game than I am.
I guess I will leave you one thought that I believe holds a certain wisdom: Language is not logical, and it is not math. (Though it has some logical elements in it.) The "rules" get broken a lot. So be fluid and have fun with it. Let the words live a little, and don't be afraid to allow the meaning to be obscure. One of the more common reasons for difficulty with foreign languages is the figurative language, so enjoy some flights of fancy with the symbolism.
Also, look at a few oddities in some of the real languages. Intonations in Navajo are very different from most, and it doesn't signal a question with a higher tone. (Nor does Japanese.) Does it have a lot of subtle sounds, like Greek, or are they very simple, like Spanish and Japanese. Are there some very unique attributes, like the pops of the Kalajari Bushmen or the odd L with a slash (you'll do better to look it up and read about it than to hear my explanation--it gets even more unusual when combined with a t.)
A few things I'm doing (sort of) are studying hand gestures, body language, and adages of other languages. (Particularly Japanese and Navajo.) I look at the word for word translations of the adages along with their meaning (In Japanese, a "sitting duck" is called a "carp on a cutting board"), then I try to form something in English that is similar, yet different. I'd like to explore a language that is not Indo-European. (The language of the Basques, I think, is one.)
Personally, though -- If it wasn't giving me a lot of pleasure, I probably wouldn't fret too much about it. I'd find other ways to show the culture is speaking another language.
Good luck!