I made this mistake in the first fantasy manuscript I wrote with my own language. I thought I could just throw out whatever and it would work because it was a made up language. Guess what? The feedback I got was something like this:
"These words look NOTHING alike. None of them."
"I like the story, but their language is really unbelievable."
"You've never studied languages, have you?"
And they were completely spot on. When your language looks like: "Xuop uenns to nkto yu kam lau'la eknataliantbrlmdpkndiflasa?" there is something wrong. (Don't worry, my language didn't look THAT bad, haha, this is just an exaggerated example.)
Look at some of the words from their language: hrududu, pfeffa, thlayli, zorn, hraka, homba, elil, fu inle.
Compare--look at some of the words from Spanish: muy, cabeza, boca, morena, nina, buenas, llamo.
Japanese: kokoro, tsuki, oshiete, wa, namae, iie, gambate, itadakimasu.
Mandarin: zhongwen, ni, qiu, ma, laoshi, dui, chun, yue, zai, xiang
Oh my goodness I could keep going! (You know by now I'm long-winded, yes? Good! Continue to forgive me.) Back to Richard Adams. His language LOOKS like a language. More than that, it FITS the characters who are speaking it--if you can't picture a rabbit with its little wiggling nose saying, "Hrududu, hrududu," then you've never stared at a rabbit long enough.
Now, I'm not saying you need to be J.R.R.Tolkien or James Cameron or whoever created Klingon. If you want to make a complete working language, FANTASTIC, if not (like lazy old me), you still need to put in the time and imagination to make the words you do use believable. It makes a tremendous difference, believe me.
(This is self-explanatory for real languages, right? I hope we all understand we must spare ourselves from the embarassment of putting Arabic in our books when we know nothing about it and having someone who does know the language corner us and demand to know why this character called his girlfriend a lovely cow.)
Current word count today: Considering we just rolled midnight into a new day: 0. I better get cracking!
Current song: Lune by Bruno Peltier [Gringoire, Notre Dame de Paris]
Current quote: "De gustibus non disputandem est." ~ Cicero