Equally of course, the story is written in English. I just write in modern day English, as the language of my target audience, and I think this is the best way to convey the setting as being somewhere the inhabitants of the story feel at home.
Most of the time, therefore, language doesn't get a mention. It is just there in the background, and not something I want to draw attention to.
However, subtleties of language do make their impression on the story.
There's the slightly stilted speech patterns I use for the key Imperial characters. Not quite enough (I hope) to draw comment, but enough (I hope) to hint at the entrenched hierarchies and ancient etiquettes Shayla has to immerse herself in.
I deliberately chose to use feet and miles for distance, rather than metric. You'd think the latter would be a natural choice for a far future spacefaring setting, but I wanted to convey, not modernity, but deep tradition and antiquity. This is a place with history.
In some of the unstated backstory, Shayla's preparation included learning not only the northern fishing dialect of the woman whose identity she steals, but also the inflection that such a dialect would bring to the standard Imperial tongue. She went further, and softened the accent to match the more refined circles her target had moved in.
There are more explicit mentions of things like the Firenzi military tongue that Shayla reverts to when talking to some captured soldiers.
The most unusual examples, though, are the secret languages Shayla and Brandt invented. Some of Shayla's security software "talks" in code by modulating the tones and rhythms of abstract meditation music. She can be listening to a detailed report while an eavesdropper wouldn't be able to tell there was a coded message in there.
At one point, Shayla and Brandt talk about this language, called Chirple, and later Brandt sends her a message using it. Here Shayla reflects on the message and its meaning:
It was said that the local dialect in Prandis had fifty different words for sunshine. The brazen midday sun in a clear sky. The caress of a spring dawn. The umber glow through an afternoon dust storm.
Chirple had equally subtle distinctions to denote varieties of intelligence. Shayla pondered Brandt's choice of wording. Fortuitous. Arrived at by chance. Neither knowingly revealed by the target, nor specifically sought by the recipient.