A translator who works with vernacular goes to bed pondering the nuance of the BCMS word dupe.
What would the story’s characters, an over-the-top father and a reticent adult son, say in conversation: ass, butt, behind, rear end, derrière, bottom, buttocks? . . . No, not buttocks . . .
Is the father, laid up in a hospital bed, asserting his virility (they cut off my leg, not my dick) by using the more aggressive ass? Or is he being playful, calling the nurse’s backside a bum? Further along the playful continuum, cheeks has possibility (later the father says something that could be humorously rendered in English as I’d like to pinch those cheeks), but this character is more straightforward than that; he does not speak in winks and nudges.
This puzzle cannot be solved by pondering alone.
Dupe appears in Benson’s Serbo-Croatian into English dictionary  (labeled vulgar) and in Babić’s Bosnian <> English dictionary  (labeled informal). The anatomical meaning of butt is not given in either my English into Serbo-Croatian dictionary (Benson ) or my English into Croatian dictionary (Filipović ), but it appears in the Babić dictionary with the Bosnian equivalent guzica. In my Random House Compact Unabridged , this meaning for butt is labeled slang. So perhaps butt and dupe are somewhat equivalent in terms of their relation to their respective linguistic standards. On the other hand, ass in Random House is labeled vulgar (not slang), and ass in Benson’s English into Serbo-Croatian dictionary (labeled vulgar on the English side) is defined as “dupe; zadnjica.”
On the other hand, Benson himself warns lexicographers and linguists that “one must be cautious about assuming the absolute equivalency of any two items in different languages.” 
Dupe appears in two of my four slang and colloquial dictionaries [7, 8], but the other two are quite restrictive, limiting themselves to terms specific to either Zagreb  or Belgrade . Dupe would not need to be mentioned in these latter two dictionaries because it is a widely understood term and not jargon as defined by these two dictionaries. I think the main reason dupe is listed in the other two is that it is useful as a headword for a number of slang and colloquial phrases.
Saving the best for last, I checked my synonym dictionary  and got an eye-full. It says that dupe is an extension of duplja — cavity, chamber, interior space. According to the entry here, at least as I read it, dupe is not considered vulgar, at least it is not specifically tagged as vulgar, whereas other terms are.
So much for terminology research.
Butt, I think, would be the most neutral choice. To me, butt is a term that a high schooler would use. A little rude, but pretty much the only choice for a teenager — or anyone else, for that matter — who doesn’t want to provoke but also doesn’t want to sound like a prude or speak in euphemisms. To my ear, ass is harsher, more vulgar. But so much of my own upbringing and sensibilities and experience play into how I “hear” these terms.
I could go further and ask friends what they think. Take a poll among my American friends and colleagues to rank five terms (only five?) in English from the most euphemistic to most aggressively vulgar and among my Bosnian, Croatian, Montenegrin, and Serbian friends and colleagues to rank five terms common to their languages. But although that would be fascinating and fun (and I might do it anyway), the decision ultimately falls to me.
The question is, will I fall on my butt or will I fall on my ass?
 Benson, M. Srpskohrvatsko–engleski rečnik, 3. izdanje. Beograd: Prosveta, 1993.
 Babić, M. Bosansko–engleski i englesko–bosanski rječnik. Sarajevo: Bosna leksika, 2002.
 Benson, M. Englesko–srpskohrvatski rečnik, 3. izdanje. Beograd: Prosveta, 1993.
 Filipović, R. Englesko–hrvatski rječnik, 19. izdanje. Zagreb: Školska knjiga, 1992.
 Flexner, S.B., editor in chief. Random House Compact Unabridged Dictionary. New York: Random House, 1996.
 Benson, M. Culture-specific items in bilingual dictionaries of English. Dictionaries: Journal of the Dictionary Society of North America, 1990, no. 12, pp. 43–54 (citing Bugarski, Hohulin, Tomaswjyk, and Zgusta).
 Saračević, N. Rječnik sarajevskog žargona. Zenica: Vrijeme, 2003.
 Šipka, D. SerboCroatian–English Colloquial Dictionary. Springfield, VA: Dunwoody Press, 2000.
 Sabljak, T. Rječnik šatrovačkog govora. Zagreb: Globus, 1981.
 Gerzić, B., and N. Gerzić. Rečnik savremenog beogradskog žargona. Beograd: Istar, 2000.
 Lalević, M.S. Sinonimi i srodne reči srpskohrvatskog jezika. Beograd: Leksikografski zavod Sveznanje, 1974.
And if you’ve made it through the references, here’s your reward: the entry in Lalević, too good not to post in full: