Hidden

The man gave her an unusually obvious glance over the points she was using for her Hidden. Rustiness from not using it in a while, or surprise at her using it? Tilt in the head that way, at that angle, with a particular shift of his shoulders and his left eyebrow lifted- [Well that’s surprising, I’m considering your claim, apologies for the scare.]
“Sorry, I thought they were back again. Can I help you?” He asked politely out loud.

Hidden, appropriately named, is a secondary language of the Locjanians. It has multiple purposes, including for all manner of negotiations. It is also used widely for insults and fighting, as it isn't considered seemly to express such things out loud (it is also one of the only places you will find hyperbole amongst them, as they will only threaten violence, not act upon it). Two Locjanians seeming to have a pleasant lunch can, in fact, be having an argument with the crudest insults and most horrific of threats being thrown back and forth without anyone of another species realising.

Practical Use

The most important and obvious aspect of Hidden is that it is entirely non-verbal. Much of it is to do with body language and positioning, which can include things like where you place cutlery or how you move your dishes in an aforementioned lunch scenario. Another highly important aspect is how specific and subtle it is; the difference between an expression of surprise and one of contempt is as little as a ten percent change in the angle of a head tilt, for instance.
For this reason, Hidden is not a language that is used at all times. It would be very difficult and cognitively taxing to pay attention to it at all times. The use of LJT-CP in their clothing means that they can turn on and off an indication that they are currently using Hidden to alert others to pay attention. Before the invention of LJT-CP necklace-like symbols that you would manually flip were used, and can occasionally be found in some places as trinkets or antiques.

Geographical Distribution

Wherever there are Locjanians, you'll find Hidden being silently spoken. Of course, this means it is almost entirely limited to the Tarinian System. Many Locjanians struggle to not do it automatically around other species, so used to it as a way of life, though on the other hand few Locjanians will ever have extended contact with other species so it isn't much of an issue.

Common Phrases

Much of Hidden has vaguer meanings that, when combined together, express a sentence's worth of information or more. They require far too intricate motions to be expressed via writing. However, some examples of the sort of things you can say in a trade include: "Peace", "I refute that", "I’m a good guest", "Open to bargaining", "More information is needed".
Insults and threats, on the other hand, can be far more inventive and strange, especially coupled with spoken speech:

“Can I help you?” Rena choked down her reaction at the Hidden that accompanied it, thankful that she was the only one to recognise the signs that essentially snarled [off a fucking tower. I’ll even get your piss-ugly coat for you, since it suits you so well.]

And finally, it can be used to express vague states or conditions, including a "Friendly face", "I have concern" and "Vicious delight", for when you've just seen someone get a very satisfying beatdown or you're looking forward to giving one yourself.