Creating a language from scratch isn't easy, no matter what anyone seems to think. The most famous example of a created language for literary use is Elvish, created by JRR Tolkien for his Lord of the Rings books. Another would be Klingon, created for Star Trek by Michael Okuda (I think I have the right name). The same man who created Klingon also created Atlantian for Disney's "Atlantis". Just a bit of trivia.
Many authors in fan fiction don't want to take the time and effort to create an entire language from scratch. We just don't have the time. Below I'll be adding some links to webpages, usenet groups, and mailing lists in case you *are* interested in doing some more than a brief splattering of words in the story. Creating a language is usually essential to world-building, which we'll get into later.
Since I have no basis on which to, well, base anything, I'll just tell you what I do. I go by sounds. I write out the words as they sound, with the appropriate dashes, apostrophes, hyphens, and accents as I want. That way all the reader has to do is sound them out and voila! That's the easy way...cheating some would say, but hey it works for me!
Another method is to get basic nouns and verbs down in your language and branch from there, adding in your additions as you go. The word for 'house' should probably always be the same word, etc while 'mudhouse' might have an extra compound in front of it. Readers today can be picky or not picky. There's never really an in-between in the extremes. They either pick apart every word you write, looking for inconsistencies or they go along with the flow and don't care that house has two different "words" for it. (grin)
Another way is to actually write out the sentence you want and "create" the non-English/your native language for it later, giving you a better molding of how you want the language to shape. With all the sentences, phrases and words in front of you that you wish to convert, it will be easier to keep things straight, concise and uniform. That way you won't have to go scrolling through the story looking for the sentence where you had "house" and then copy it. If you mark in your word processing program somehow where each sentence goes (with a number sequence for easy Find), you can cut and paste in your "converted" sentences, phrases and words without too much difficulty.
Another way, which many just do to avoid the hassle of the above, is to put the dialog in special bracketing to indicate its in another language and merely note to the reader in the story headers that {} means the Such-Such Language. The problem I always have with this is that so many stories have thoughts and telepathic dialog that all the brackets are used up. And readers will get tired of deciphering that [] is telepathy, {} is thoughts and ~word~ is a made-up language. If you make it too confusing for the readers, they won't continue reading because it gives them a headache.
Is there a way around that? I'm sure there is, but it takes a lot of finesse, good writing and the ability to work with words and the made-up language. Your best bet is to do this (which is a cop-out I know, but eh, I didn't say you *have* to do it this way):
"Why is it in your pocket?" Migue said in their native tongue. The other three looked at the two Shileans, not understanding what had been said.
Tada! Cop out! But it works.
It's all dependent upon style and the context of the story, really. And personal choice. You also have to know your readers. Are they the type that would get a kick out of seeing an "original" language for the characters? Or would they prefer just to be told the words are in the character's native tongue?
The key, as slightly mentioned above, is in moderation. Don't overflow the story with the language. Just a bit between characters or groups of characters every once in awhile, to give the story and their culture flavor. Use it as a plot point, a secret being kept away from others.
Above all, have fun with it. If it's not fun, you must not be doing it right.
I know all of this sounds vague, and I haven't done a good job with this one, but hopefully this and the links below will give you a good starting point to work beyond my dribble here.
Model Languages & The Art of Language Making
The Random Vocabularies/Name Generation Page
The Elvish Linguistic Fellowship
Ardalambion: of the Tongues of Arda, the invented world of JRR Tolkien
Newsgroups:
alt.language.artificial
Mailing Lists:
Gornog - this group seems to be attemping to create a new language
Shadows With Eyes - seems to be members only and I have no idea, but they seem to be creating a new language
DutzProject - The project working on language, life-forms, and others things to create a great science-fiction creation.