On planet Gereon Ghosts meet "partisans" that speak language other than Low Gothic. After a bit of concentration, Gaunt recalls that he was learning something similar in scholam
Gaunt’s brow creased as he concentrated. He could almost see old Boniface now, smell the musty scholam room, Vaynom Blenner at the desk beside Gaunt, doodling cross-eyed eldar on the cover of his slate.
“No looking at your vocab primer now, Scholar Gaunt,” Boniface called. “Parse the verb form now, young man! Begin! ‘Ayeam yclept… Heyth yclept…’ Come on, now! Blenner? What’s that you’re drawing, boy? Show the class!”
“Histye, soule,” Gaunt said, more deliberately now. “Ayeam yclept Gaunt of Tanith His Worlde. Preyathee, hwat yclepted esthow?”
“Cynulff ayeam yclept,” the partisan replied. “Of Geryun His Worlde.” His voice dripped like glue in the sweaty air.
“Histye, Cynulff,” said Gaunt. “Biddye hallow, andso of sed hallow yitt meanye goode rest.”
There are more examples later on, i.e:
“Preyathee, soule,” Gaunt asked, tapping the bow. “Hwat yclept beyit?”
“Reynbow, beyit,” Eszrah replied. He was delicately picking meat from a spindly frog-bone with his small, white teeth.
“Reyn-bow, seythee?” Gaunt repeated. Eszrah nodded. “Thissen brande sowithe yitt we shalle reyn yron dartes thereon the heddes of otheren kinde, who gan harm makeyit on us.”
A rain-bow, to rain quarrels on the heads of the enemy. Gaunt smiled. He’d seen that. Emperor bless the nightwalkers.
While of course this might be completely fictional language, it does sound look similar to Middle English. Could someone with more linguistic skills confirm or deny it?
Edit: TvTropes describe it as an example of "Ye Olde Butcherede Englishe":
Used in Traitor General to represent the old, proto-Low Gothic language used by the Nihtganes. Although there, it was a combination of normal Butcherede Englishe, random "misspellings" that make everything look like it's weirdly pronounced and some words that Abnett just plain made up.
So I guess question is answered.