GERMAN MEANING
It is assumed that the categorisation is accurate: “Bauplatzvorbereitung für den Hochbau” (preparations for high-rise construction). So I begin by searching in that context:
https://www.bing.com/search?q=Hochbau Sauberkeitsschicht

“Werden die Fundamente bewehrt ausgeführt, wird vor dem Aufstellen der Schalung auf dem Baugrund eine Sauberkeitsschicht aus z. B. Beton C8/10 eingebaut, die als Auflage der Abstandhalter der Bewehrung dient.”
“Zunächst ist eine Sauberkeitsschicht aus ca. 5 cm Beton C8/10 einzubauen. Dann erfolgt das Setzen der Randschalung und der Einbau der Bewehrung. Anschließend kann der Beton für die Fundamentplatte eingebracht werden.”
https://www.beton.org/wissen/wirtschaftshochbau/fundamente/

“Die Begriffe Sauberkeitsschicht (kurz SKS) oder Unterbeton bezeichnen im Bauwesen eine dünne Magerbetonschicht, die sich zwischen der Baugrubensohle und dem Gebäudefundament befindet.”
https://de.wikipedia.org/wiki/Sauberkeitsschicht
“Magerbeton ist ein erdfeuchtes Kies-Zement-Gemisch im Verhältnis 8 : 1.”
“Man verwendet Magerbeton hauptsächlich für Füllungen, die druckfest sein müssen, für Sauberkeitsschichten, zum Ausgleichen von Unebenheiten im Untergrund, als Schutzschicht über/unter Dichtungen, Verfestigungen und Verfüllungen. ”
https://de.wikipedia.org/wiki/Magerbeton
“Die Baugrubensohle bildet den unteren Abschluss der Baugrube und dient als Gründungsebene für das zu errichtende Bauwerk. [...] Bindet die Baugrube in das Grundwasser ein, so kann die Baugrubensohle durch Injektionssohlen oder Unterwasserbetonsohlen abgedichtet werden.”
https://de.wikipedia.org/w/index.php?title=Baugrube&stable=0...

ENGLISH DESCRIPTION
“a thin 3" layer of plain cement concrete (PCC) is poured just to create a perfectly flat and level base for the foundation.”
http://www.understandconstruction.com/raft-foundations.html

As the Sauberkeitsschicht is a layer located beneath other layers/materials and prepares the site for further construction, it could be described as a “preparatory layer”, a “sub-layer”, an “underlayer”, a “substratum”, etc.

APPLICABLE ENGLISH JARGON
“Blinding concrete is a layer of (usually) lower grade, possibly unreinforced concrete placed on the ground. It serves to isolate the structure and/or users above it from the soil below, and also to provide a stable, potentially flat surface above which to launch the construction of a structure.
The structure above may consist of a slab placed directly onto the blinding slab. In which case, the blinding concrete provides a consistent surface off which to chair reinforcement.”
https://www.quora.com/What-is-blinding-concrete

“A blinding concrete plate is a base layer that is cast into a footing trench before installing any reinforcement for foundations. It has a minimum thickness of about 2 inches.”
“Blinding is used to create a strong level surface for either of the two purposes: an underlying mass fill and a concrete slab.”
https://www.reference.com/home-garden/blinding-concrete-plat...

“In construction, blinding is a base layer of weak concrete or sand that is laid above a layer of hardcore to provide a clean, level and dry working surface.
A thin layer, usually around 50 mm (2 inches) thick, of blinding is poured over the hardcore, sealing in the underlying material and levelling off the surface.”
https://www.designingbuildings.co.uk/wiki/Blinding
Notably the blinding layer is more stable, more level and less permeable than the underlying material.

“Blinding concrete” is also used in Australian English. They also contrast that with “sand blinding”.
https://www.strathfield.nsw.gov.au/assets/Development-Applic...

“Mud slab” may be applicable. I do not take credit if so.
The following definition seems a bit too specific to suit:
Mud slab: “A layer of concrete, 2 in. (5 cm) to 6 in. (15 cm) thick, below a structural concrete floor or footing over soft, wet soil.”
https://encyclopedia2.thefreedictionary.com/mud slab
Similarly on Wiktionary
“ A layer of concrete, typically 2 to 6 inches thick, poured below the structural slab but above a layer of wet or muddy soil.”
https://en.wiktionary.org/wiki/mud_slab

Conversely, “mud slab” might refer to a slab intended for light use only.
“A mud slab refers to a minimum thickness, slab of concrete, that normally does not have any reinforcement.
This concrete slab is meant to cover the dirt of a basement, or basically clean up the surface of a floor, in an area that will not obtain much use. [...] In projects that have problem soils or [severe] water issues, the use of the mud slab to establish a working platform is sometimes specified to maintain the structural integrity of the base of the footings. The wash out from the concrete trucks on large projects [may be] enough concrete to create a useable mud slab for exterior storage or a temporary parking area.”
https://www.builder-questions.com/construction-glossary/mud-...

“Been building here in New Zealand for the past 3 years.The foreman [...] asks me what a “mud slab” is. He [...] reckoned it was some American term.I said I’d never heard the term.Spent 5 years in the foundation and flatwork business exclusively(4 in Connecticut,1 in Oregon) [...].Never heard the term there either.”
“As you guys have described [...] a mud slab would be a facilitation technique and not something that [...] should be specified in plans.”
“Yes, right. It's a response to unpredicted, and likely unpredictable, site conditions. And in our case, it involved a discussion with the engineer and inspector--just to be safe.”
“Kinda rare around here but when the term is used it tends to be any non-structural slab poured cheaply, typically thinner and with less, if any, reinforcement, and primarily for the convenience of having a cleaner, drier and flatter surface to work on.”
https://www.finehomebuilding.com/forum/whats-a-mud-slab
So “mud slab” is not necessarily a common term even in the USA.

Poster ‘BigH’ (a Canadian with experience working in Asia) seems to prefer the term “blinding concrete”, which they take as a synonym for “mud slab”.
https://www.eng-tips.com/viewthread.cfm?qid=211074

tazdog quotes the Penguin Dictionary of Civil Engineering:
mud slab, mud mat - ACI's terms for the UK's blinding concrete. (ACI = American Concrete Institute).
https://www.proz.com/kudoz/spanish-to-english/construction-c...

“Mud slabs, also known as rat slabs, are thinner than the more common suspended or ground-bearing slabs (usually 50 to 150 mm), and usually contain no reinforcement.[...] In general, they may be used for any application which requires a flat, clean surface. This includes use as a base or "sub-slab" for a larger structural slab.”
https://en.wikipedia.org/wiki/Concrete_slab#Mud_slabs

Definition from the American Concrete Institute:
“mud mat — a layer of concrete beneath a structural concrete floor or footing over soft,
wet soil.”
https://www.concrete.org/portals/0/files/pdf/ACI_Concrete_Te...
No mention of “mud slab”, nor “blinding layer/concrete/slab”.

INAPPLICABLE ENGLISH JARGON
“Granular subbase” is for road building, laying pavements, or pipes, and refers to gravel-like material.
https://dict.tu-chemnitz.de/deutsch-englisch/Sauberkeitsschi...
Similarly, “hardcore”, but which could go underneath the blinding layer.
https://www.designingbuildings.co.uk/wiki/Hardcore
See also “aggregate” and “gravel”.
https://www.designingbuildings.co.uk/wiki/Gravel_v_hardcore_...
“According to the ACI Code, the subgrade is a compacted and improved natural soil or brought infill whereas subbase is a layer of gravel placed on the top of the subgrade.”
https://theconstructor.org/practical-guide/subgrade-subbase-...
See also https://en.wikipedia.org/wiki/Subbase_(pavement) & https://en.wikipedia.org/wiki/Subgrade
and https://theconstructor.org/transportation/concrete-pavements...

Maybe subbase/sub-base can be appropriate in some applications ...
https://s.bega.com/fp-0227442232069938688830/media/pim/impor...
... but not necessarily in high-rise construction

“Flowable fill” seems to have a different purpose.
“Underconrete” doesn’t seem to be a conventional expression.
https://www.proz.com/kudoz/spanish-to-english/construction-c...

TRANSLATIONS
Leo has:
subbase [context unstated]
blinding concrete [TECH.]
blinding [GEOL.]
granular sub-grade course (in the bottom of the trench) [TECH.: Pipes]
https://dict.leo.org/german-english/sauberkeitsschicht
The respective contexts are important.

And also has a discussion
https://dict.leo.org/forum/viewUnsolvedquery.php?idforum=1&i...
“A blinding layer of lean/plain concrete” is OK, albeit a little wordy.
“Cleaning layer” was a guess based on deconstruction of the morphemes that doesn’t work in English.
“Subbase course” is wrong, as it is two layers beneath the actual concrete. Note the question mark inserted by the respondent.

An English–German Bau Lexikon hosted on a German website has “blinding layer”.
https://www.agtze.uni-hannover.de/fileadmin/agtze/pdf/Woerte...
Although unstated, this is apparently British/Commonwealth English, in view of their spelling of “armour” and “behaviour”.

Another English–German technical glossary hosted on a German website has “blinding”.
http://www.abg-plus.de/abg2/dok/WoerterverzeichnisEngl-dt.pd...
This is evidently written in American English, because it is credited to “US ARMY ENGINEERING DIVISION, EUROPE. CORPS OF ENGINEERS”, and because it includes spellings such as “labor” and “odor”.

FRENCH
"Couche de propreté" or “béton de propreté” may correspond, from very brief inspection.
https://www.isotosi.ch/sites/default/files/iso-tech-2015-11-...

RECOMMENDATION
“Blinding layer/slab/concrete” seems feasible for use in either the UK or the USA (loosely speaking: in any English variant). “Mud slab” or “mud mat” appears suitable only within the USA.
“Blinding concrete”+ construction + high-rise. UK: 21. USA: 90.
https://www.google.com/search?hl=en&as_q="high rise" constru...
https://www.google.com/search?hl=en&as_q="high rise" constru...
“Mud slab”+ concrete + construction + high-rise. UK: 10. USA: 104.
https://www.google.com/search?hl=en&as_q="high rise" constru...
https://www.google.com/search?hl=en&as_q="high rise" constru...
“Mud mat”+ construction + high-rise. UK: 10. USA: 111.
https://www.google.com/search?hl=en&as_q="high rise" constru...
https://www.google.com/search?hl=en&as_q="high rise" constru...


--------------------------------------------------
Note added at 51 days (2019-09-22 02:25:50 GMT) Post-grading
--------------------------------------------------

A schematic image, although it doesn't add much:
https://www.onlinebaufuchs.de/abload/img/0520181101353.JPG