From Discworld & Terry Pratchett Wiki
A spiky and heartfelt profanity in an Überwaldean language, drawn out of Otto Chriek in a moment of stress when he lost an Überwaldean Land Eel in the cellar of the Ankh-Morpork Times offices. While he apologises to Sacharissa Cripslock and describes the incident as being a minor pothole on the road to progress, we are left in no doubt that serious swearing has just happened, whilst left in the dark as to what physical processes or minor blasphemeies have just been voiced.
Interestingly enough, while running the phrase through various translation engines in Slavonic languages has nil result, it does bring up a scholarly discussion with the Czech translator of Pratchett's books, Jan Kantůrek, in which he discusses how he went about translating cod-Slavonic swearing into a Slavonic language. 
"He explains in the note that he simply could not deprive the Czech reader of the original Pratchett's sentence (Kantůrek, Pravda p239). An objective explanation of why he used the original sentence is that it has not got any meaning whatsoever, so it would in fact be impossible to translate it even if Kantůrek tried. However, as this sentence was pronounced by a vampire, a foreigner in fact, and moreover, it should have meant nothing else that a swearing, it does not seem improper in the text, even though it is not in Czech"
The suggestion is that as the speaker is an uncouth foreigner and not to be confused with a Czech in any way shape or form, the cod-Slavonic could safely be left in place to suggest Otto is a Pole or a Russian and can therefore be as foul-mouthed as he likes, why are we not surprised?