There are several ways to make your writing authentic, but the one I would like to touch upon is the way characters speak. 

Mixing English and another language is not anything new or groundbreaking in literature. Take Agatha Christie's detective, Hercule Poirot. 

To begin with, Poirot is not French, he's Belgian, but of the three most commonly spoken languages in Belgium, Poirot speaks French. In Agatha Christie's The Affair at the Victory Ball, Poirot says,  "He [Hastings] was just holding forth on the subject, n'est-ce pas, mon ami?" 

Context, for the purpose of this example, does not apply. What does apply is how Poirot has a proclivity to mix his French in with his English; it is one of his most memorable traits. Language can help tell a story, it can add immersion and authenticity, but it can also, in this instance, tell the story of your character without going too deeply into it. This sort of detail can help writers avoid the "need" to add unnecessary backstory or explanations for your characters sometimes. 

The next aspect of language is jargon, slang, specifically colloquialisms. 

What's the difference between the two? 

Slang, according to the Oxford Dictionary, is "a type of language used by a particular group of people - e.g. teenagers - or by people in a particular occupation - e.g. members of the army. It can act as a kind of code: using slang creates a sense of identity or belonging among members of the group and effectively excludes outsiders." 

Slang can help differentiate one character from another or one group of characters from another group. It can speak volumes about age and background. For example, most people say "gossip," while a number of teenagers and young adults now use the word "tea" when referencing gossip. It seems silly, but it does in fact reveal a lot about the gossip. 

"Tea" or "spilling the tea" (a play on "spill the beans") refers to "fresh or hot" gossip. 

As for colloquialism, it is defined as "a word or phrase that is not formal or literary and is used in ordinary or familiar conversation." 

An example of colloquialism is, as I said in the first part of this article, the word "hella." It isn't slang, it isn't short for "hell of," it is just the way people say it in a certain part of California. Other more commonly used colloquialisms are the words: gonna, wanna, and Y'all. 

They are not slang words, they aren't short for anything, they aren't jargon. These are words that are used as naturally as the words they originated from. A colloquialism is a helpful tool when it comes to identifying where certain stories take place without revealing the location. Just like Poirot's speech pattern, using colloquialisms can also hint at a character's background without having to dive deeply into it. 

According to the Cambridge Dictionary, the word "jargon" is defined as "words and phrases used by particular groups of people, esp. in their work, that are not generally understood." 

If I were to say, "chest candy," the general image in people's heads would be a chest with candy on it. Now for people who are or were in the military the term would be understood; it simply means ribbons and medals that are worn on a uniform. 

Jargon can be code and also viewed as an exclusive form of slang sometimes. Jargon is often used in professions like the military, journalism, politics, and the list goes on. Like everything else I've talked about, jargon can reveal things about your characters, like where they work, their past, and so on. 

Jargon, slang, and colloquialisms can be an integral part of storytelling and characterization. It might not be necessary for every story, but authenticity in language can go a very long way. It can be a minuscule detail that reveals something about a specific character or it can be a tool used to create an authentic atmosphere.