We have recently added 4 more authors to our wagon--welcome! You can visit them to see how they apply languages to their work:
Anastasia V. Pergakis, J. D. Brown, Alex J. Cavanaugh, Deirdra Eden-Coppel, M. Pax, Jeffrey Beesler, Charlene A. Wilson, and Andrew J. Cooper!
I love to invent words. I have a list of made-up words in my writerly sketch book. My first novel is a high fantasy read using invented words for native creatures. The goblins have their own language, which I didn't translate. Oops.
Nowadays, I keep a dictionary in a separate file and refer to it as I write the novel.
I keep my words simple, pronouncable. I don't always directly translate what a character says:
"Fermer la porte." Collette shivered.
"Do you mean close the door?"
"Fermer la porte," Collete pointed to the door with a shiver.
"You get cold too easy, I swear." John kicked it shut. "You're welcome," he taunted.
Like in the second example, I drop clues so you won't catch a headache. I do the same with real languages such as my Viking novel and Eros.
How do show your character language?