Sjandra Kei. Sjandra Kei is the "New Earth" sort of planet, the place where most of humanity lives. It is in one of faster zones of thought where faster than light flight is possible, but nowhere near The Transcendent.
There is also "The Old Earth", our own planet, 3rd rock flying around Sol star from which humanity came from. The problem is, 1) nobody now knows where "The Old Earth" is supposed to be and its location now lives only in memories and legends, sort of like Atlantis for us 2) it is in the slowness zone, where it is impossible to fly faster than the speed of light so nobody can get in and out in a lifetime to look for "Original Earth". More than one expedition was lost in an attempt to find it.
The thing you might be more familiar with is "Sjandra Kei Commercial Security Fleet" which is used as an answer in this question.
Basically, what happened is:
There were multiple calamities on Old Earth. There was age of space exploration and "Age of Broken Dreams". Age of Broken Dreams is so called because humans realized that FTL travel and artificial intelligence and some other technologies are impossible. Turns out, they are impossible because of galactic Zone they are in, which is one of Slow zones.
One offshoot of humanity finally got out of Slowness zone after untold millennia of travel and built their home on Sjandra Kei, where all those "impossible" things are actually possible. Most of humanity outside of Slowness zone is Sjandra Kei humanity. Pham Nuwen is one of exceptions.
Basically SjK units are "humanity language units", sort of like English is trade language on most of our planet now, despite there being multiple "nationalities" on Sjandra Kei with their own language (and we meet some of them in SjK Commercial Security Fleet), Sjandra Kei trade language is considered to be default human language. Also, if I understood correctly, Triskweline and Samnorsk are some of local Sjandra Kei languages.
So, basic meaning of this is:
Firetongue->Cloudmark->Triskweline, SjK units [Firetongue and
Cloudmark are High Beyond trade languages. Only core meaning is
rendered by this translation.]
Firetongue was translated to Cloudmark, Cloudmark was translated to French (or Norwegian, or Japanese or whatever local language you choose) written in English notation.
Like this: "なの" written as "nano" where "なの" is Triskweline and "nano" is SjK units.
And difference between SjK units and SjK Relay units: SjK units are your full language units. SjK Relay units is simplified notation made for trade and business deals. Sort of like Chinese (Traditional) and Chinese(Simplified). It's the same notation, just uses subset of full signs.
And that's one of the reasons I absolutely love "Fire Upon the Deep".