How about "inu no kintama" for the Japanese translation. 犬の金玉. Literally "dogs gold balls".
I have a copy of "Japanese Street Slang" at home. As you might imagine, it has a whole section devoted to testicles :) Kintama is the #1 word they recommend, but (as with many of the words in the book) I've never heard it spoken. It does seem to have a good pedigree, though (no pun intended).
The other word that I was actually going to suggest is in there too: O-inari. The 'o' at the start is an honorific prefix. Look up the web to see pictures of "Inari sushi" (contracted to "inarizushi"). For example, this one. From the resemblance to scrota, it should be obvious that people could understand its slang use. The only thing about using it with kitsune (as opposed to inu) as some people have suggested is that there's also a kami (somewhere between a spirit and a god) named Inari, and the kitsune are his messengers. If you said something like 'kitsune no inari", people might thing you were referring to the kami and get confused. You'd just have to try it out on a native Japanese speaker.
On another related point, I'm sure loads of you have heard the story about how the dog's bollocks could have come from Meccano sets and the "Box Deluxe". I'm sure that's pure bollocks. I always thought that the idea came because there must (self-evidently) be something good about them (the dog's bollocks) because they like to lick them so much. I've always wondered if the phrase translated literally into other languages because it would be strong support for the "self-evident" etymology theory. When I heard 'la puta madre' in Spanish I thought it literally meant 'dog's bollocks' (madra being the Irish word for dog, but that's a false friend). Alas, although it does translate to it in English, it's not a literal translation.
Anyway, this is all vital research, and I'm glad that el Reg is championing it in its pages. Thumbs up!