English version:
A zookeeper wants to install a mongoose exhibit. So he writes to his animal supplier. "Dear Sir, please send us two mongooses - " he writes, then scratches it out. "Dear Sir, please send us two mongeese - " That doesn’t work either. Finally he writes, “Dear Sir, please send us a mongoose. P.S. While you’re at it, send us another one too.”
French version:
Le curateur d’un zoo veut monter une exhibition sur le chacal. Alors il écrit à son chasseur: "Monsieur, veuillez nous envoyer deux chacals - " Il l’éfface puis l’essaye encore une fois: "Monsieur, veuillez nous envoyer deux chacaux - " mais cela ne marche non plus. Enfin il écrit tout simplement, “Monsieur, veuillez nous envoyer un chacal. P.S. Ensuite, envoyez-nous-en un autre.”
(There is as yet no agreement in French whether the plural of “chacal” (jackal) is “chacals” or “chacaux”, like cheval-chevaux. Adapted from: Yaguello, M., Cataloge d’idées reçues sur la langue.)
So think of a parallel problem in a language you speak, and come up with a translation of this joke around it. Have fun!