I was talking with some people today about how many words in the English language are specialized; they're only used by small fractions of the population. Like "odontoblast," "jabiru," and "gallet," from the worlds of dentistry, ornithology, and masonry, respectively. Many occupations have their own special languages. Geologists might talk about chert, gneiss, and the vadose zone, while doctors speak of tachycardia, cyanosis, and the corpus callosum. In fact, that's one of the difficult parts about writing characters who share a profession if you're an outsider to that profession: getting the language right, whether your setting is a hospital, a restaurant kitchen, a military base, a police station, or a dance studio. (The other challenge is to keep it intelligible to those readers who are not insiders.)
books for children and teens can often skip this problem, because our
characters usually don't have these occupational vocabularies. But
sometimes our characters do live in specialized worlds--if they're
Olympic gymnasts, for example. And of course there are other vocabulary
issues, like slang and regionalisms.
Another challenge in getting
the vocabulary right is in writing historical novels. I'm always
fascinated by novels written in the 1920s, with the characters'
references to "flivvers" and "runabouts" and "berries" and
"brilliantine," and the frequency with which the phrase, "I'll tell the
world!" is uttered. A time traveler from the early 1920s would be
puzzled by our references to "surfing the web" or "texting" or "TiVo,"
not to mention "MRIs," "in vitro fertilization," or even "penicillin."
What "language" do your characters speak?