(I've always wanted to write a character who uses lots of business/office jargon and have a book called Ducks in a Row: An A- Z of Offlish by Carl Newbrook (Offlish being jargon for Office English) which yields all manner of goodies from Blue sky thinking to Running a flag up the pole. The nearest I've come to it was Marcus in Kissing Mr Wrong. It was a small moment of personal triumph to get 'athermal birefringent filters' into the text.)
The trouble with jargon is when it becomes incomprehensible to outsiders. I haven't a clue what 'athermal birefringent filters' are, but nor does viewpoint character Lu and it's not important to understanding the text. There's nothing more frustrating than being deep in a story and then getting dragged out by not understanding a particular word. Jactitation is a great word, but I don't imagine many people know what it means (restless tossing in illness, twitching or convulsion) let alone a jactitation of marriage (the pretence of being married to another). Every time the reader goes out of a story, you potentially lose them.
Foreign language is another form of jargon. All jargon potentially excludes readers, but using foreign words potentially alienates them. Your French/Greek/Latin/whatever may be fluent, but mine certainly isn't and I hate reading bits of foreign language in a novel which I don't understand. Providing a direct translation is convenient, but breaks the illusion that the novel is real life. Make the meaning of your foreign words clear from the context. These are from A Single to Rome.
'Buon appetito,' she murmured as she put a plate down in front of Natalie.
'Ciao, Natalie, come stai?' It was Teresa.
'Va bene,' Natalie said, pleased to have at least mastered the polite exchange of greetings in Italian, but then had to lapse into English.
Alternatively, have your characters translate for each other (and the reader).
'I used to be a lawyer,' Natalie said.
Bettina looked puzzled. 'Avvocato,' Claudio chipped in, handing out drinks.
Later on, I used the word avvocato without translation, hoping that it's stuck in the reader's mind.
Finally, don't forget that using jargon isn't rocket science.