Growing up, I didn't speak Spanish all the time, though my mother is a first-generation Mexican-American who speaks Spanish fluently. Although she didn't actively teach my brother and me Spanish, she did use certain phrases and worked their way into our family's lexicon, just because in her mind, it was faster or easier to get the point across in Spanish than in English.
By this point, I feel comfortable saying that I can read and write Spanish fluently, but I don't speak it all that often. Even so, there are some of the same phrases of my mother's that I hear coming out of my mouth when I'm not thinking. It's easier and faster for me to say "fíjate" than it is to say "look at that!", even if I didn't actually know that's what it meant in English. Even though my husband doesn't speak Spanish at all, he's learned what some of the phrases mean just through the way I use them. Similarly, I've picked up on a couple of phrases in Japanese, because my husband uses them as shorthand.
In general, we all have our own personal shorthand that we use with people we are close to. Inside jokes are the most obvious example of this, as anyone who has been the third wheel with a couple who has been together for a long time can tell you. Jargon's another way of describing language used for a specific purpose, as certain terms will me different things for different companies or businesses. But even when it's not some sort of joke, everyone finds a way to reference a complex experience without describe the entire event, and everyone has a way of speaking that they use only when speaking to certain friends or family.
When creating a character, it makes sense to find out how your character thinks, and figuring out what their linguistic shorthand is can be a great way of getting an insight into that thought process. Say your character has to tell someone that they are seriously ill. How would your character tell his or her best friend? What about their parents, or their employer? Imagining how the same conversation will happen with different groups of people will tell you a lot about how your character interacts with different environments, and more options can always lead to more interesting interactions and plot lines.