An option is an accent. I frequently use varying degrees of slang in my stories; it's my way of indicating someone who might have had less education or comes from 'the other side of the tracks', so to speak.
But there's always the fictional option to languages. The elven language in Lord of the Rings has evolved into a full-fledged spoken language, as I understand it, as has the Klingon language from Star Trek.
You don't need to create an entire language in order to use it. Throw a bunch of sounds together and tag it with a meaning. Of course it's important that you remember how to spell that word and exactly what that meaning is. It's also important to give a thought to how languages are used. In some cultures the family name is said first followed by the given name. In some languages verbs are come first.
I'm no English major, and I was terrible at English in high school, but that's no reason not to listen. If you're going to create a language, make what you want to say roll off your tongue. If you can't say it, your reader won't be able to sound it out either, and if your reader can't sound it, he will skip over it. Skipped, it's meaning might be missed or confused with some other word you've created. That book with the French phrases; in my opinion there were too few translations and the usage wasn't clear enough. Most of us would probably recognize some of the more common phrases, so if you're going to use made-up words, translate them in some way. If you're going to use entire phrases, all the words can't be long, flowery, collections of letters. In many languages, a long word might just be an entire phrase after translation - look up some German words someday, they're interesting.
Another thing to consider is the sound you create. Say something out loud. Yell at the kids for a second. What does the sounds you just made sound like? Make believe you know have any idea what the words mean; what kind of feeling did you just generate? To make your words or phrases have a feeling, you need to feel them, even if they only last for a single phrase. Who knows, maybe your few words will someday become a new language.
So how do you use language? Do you make something up? Do you think you could? Try it.