Nilfgaardian is a language based on the Elder Speech and used in the Nilfgaardian Empire, originating from the Nilfgaard proper itself.
It is characterized by long, drawn-out syllables
and hard pronunciation.  Some words of the classic Elder Speech have a totally different meaning in the Nilfgaardian: for example,  y which is of in classic Elder becomes and in Nilfgaardian or white, usually translated as gwen by elves while wen by Nilfgaardians.
A B C D E F G H I J K L M N O P Q R S T U V W X Y Z Phrases
 attack (from Dutch: aanval)
dance; dancer; dancing
to provoke / to disturb
from, to, for, with, of, in, out of, at, by, down, in, into, until, on, like, as
of (mostly used for marking hereditary ties)
arse (UK), ass (US), butt, rump (Dutch: aars)
important, something important (From Dutch: belangrijk - pronounciation is nearly the same)
wolf (From Welsh: blaidd)
bloody, gory, damn (UK: bloody hell / Dutch: Bloederig = bloody)
tomb, grave, barrow
empire (of Nilfgaard)
must, necessity, obligation, inevitability, duty
devil (From Spanish: diablo)
 honor (from Dutch: eer)
necessary, must, have to (From Irish: is éigean)
to be, is (From Italian: essere)
will be, will (From Latin: esse)
ambassador (From German: Abgesandter)
gláeddyv (plural: glaeddyvan)
sword (From Welsh: cleddyf)
 glory (From French: gloire)
river ( aevon in Elder Speech)
 tenacious one
 bloodthirsty one
greeting (long life to...); health (From English: hail, hale)
unit, armed group of friends, gang (From English: hanse)
 to search
 Emperor (From German: Kaiser)
daughter / young girl
 or (From Irish: ná)
neén / ne
demonym for Nordling
 needle (From Dutch: naald)
pestilence, plague, blight (from Dutch: pest)
 cocksuckers (from Dutch: piemellikkers)
that / what (From Italian: che, French: quoi/que)
 to move (From Irish: Rachaidh)
 to fall back, retreat
 scumbag (From Dutch: rotzak)
 coward (From Dutch: Schijtlijster, literally little shitbird')
 victory (From Dutch: Zege)
 dammit (From German: Scheiße, shit)
 shit (From Dutch: Stront)
time, age, season (From Dutch: Tijd)
tuvean / tuv'en
two (From Dutch: twee)
to hope, to trust (From Dutch: vertrouw)
 what? (from German: warum / Dutch: waarom)
fast, quick; quickly (From Italian: veloce)
further, away (from oneself), still (From German: fort)
white (From Welsh: Wen/wyn)
and (perhaps from Spanish: y)
on, over, upon, one
Aâ'anval neen orde!
Attack at will!
Aecáemm aen me
Aen Ard Feainn
By the Great Sun
Aen iarean nyald aep kroofeir
Like searching for a needle in a haystack
A d'yaebl aép arse
A devil up your arse
Bloody plague (imprecation)
Deithwen Addan yn Carn aep Morvudd
The White Flame Dancing on the Barrows of his Enemies
E'er y glòir
Honor and glory (usually used as a greeting)
Drop the swords!
Gloir aen Ard Feainn
Praise be the Great Sun
Glòir aen Ker'zaer
Glory to the Emperor
Long live the Emperor
Larwm! Essea tuv'en a'vean!
Me hen aveen'eans aep tooraen. Me peuvt aen alvear'daeg
I am too old for wrath. All I feel is simple grief
Me voel'tocht aen geehaet va'en. Me vaer'truov, het aen belean'graec
I had a hard journey. I trust it is something important.
Do not dare fall back
N'ess aedragh a me
Do not provoke / disturb me
N'ess tedd a thu
I have no time for you
ne'esse a fydd
Ne tuv'en que'ss, luned!
Do not kill yourself, girl!
Que'ess zaighderan aep Dravograd vaen?
How many battalions are marching on Dravograd?
Que suecc's? / Que aen suecc's?
What's going on?
Se'ege na tuvean
Victory or death
Tuvean y gloir
Death and glory
at ease, keep calm
Visse deien aep Ceas'raet. N'et aen vaerb'tinnea, het aen creasa
You serve the Empire. It's not your good will, it's your duty
Visse ess'spaet aep uav raet, daar'tocht aep Nordling daerienn
You spat on your country, conspiring with Nordling sorceresses
Visse gead'tocht gaedeen
You have done well
Nilfgaardian differs from the classic Elder Speech in that much of it seems to be based on Dutch.
In the expansion, Blood and Wine Nazairi soldiers use a great number of Dutch profanities; these include rotzak ("scumbag"), schijtlijster ("coward"), stront ("shit") and piemellikkers ("cocksuckers"). They also use the word kusse, later explained by Regis to mean vagina (akin to Dutch kut and its older form kutte, which carry the same meaning, though it more likely came from Danish where the word means "pussy").
After being asked by a fan, Mateusz Tomaszkiewicz who works as a narrative designer for CD Projekt confirmed Nilfgaardian used in their games is a mix of Elder Speech and Dutch.
The Lady of the Lake
Time of Contempt ↑
3.00 3.01 3.02 3.03 3.04 3.05 3.06 3.07 3.08 3.09 3.10 3.11 3.12 3.13 3.14 3.15 3.16
Gwent: The Witcher Card Game ↑
4.0 4.1 4.2 4.3
Thronebreaker: The Witcher Tales
The Witcher 3: Wild Hunt ↑
6.0 6.1 6.2 6.3
expansion Blood and Wine ↑
Mateusz Tomaszkiewicz about Skellige jargon and Nilfgaardian