I've encountered opinions on the manga and anime message boards crying, "Bad translation," when a Japanese curse word is not translated for an English curse word. That isn't true. From a practical, professional, and philosophical standpoint, one should be very wary about using offensive language in manga and anime translations.

Here's the philosophical backing to my assertion.
There are no Japanese curse words so bad you can't say them on television. In other words, there are no words in Japanese that are so offensive that people would complain to networks, publishers, sponsors, etc. The Japanese aren't offended by their expletives. Americans are. Big difference, especially in pop-culture translations.

Here's the practical point for translators.
Your publisher will set standards for the strength of language you can use. If you try to break their rules, you may not get hired again. (Not only that, but they'll change the translation anyway.)

So generally the problem isn't one of mistranslation, but one of personal word choice. If a translator is faced with a word like kuso, and if "Sh*t," or, "Damn," is too strong for the age group the publisher is shooting for, the translator doesn't have to resort to hokey curse substitutions such as, "Darn." There are actually a wide variety of powerful words that can be substituted -- all it takes is imagination.

"Dammit," can sometimes be used instead of, "Damn," since it carries a lighter tone. "Crap," can sometimes be used depending on the publisher. "Oh, hell," is good for looming-danger expletives. A lighter-touch expletive for that situation and for disappointment expletive circumstances is, "Oh, man!"

I've found that with the right character, "You cretin," can work as a yarô substitution when "You bastard," isn't allowed. Others, depending on situation and character (and publisher) are, "You ass," "You philistine," "You pig," or if you have room, "You stinking pile of trash," or "You festering sore!" (There are other powerful garbage- and illness-based insults available.) There is also the option of not-quite saying the curse word, such as in, "Son of a--!"

For nameru-na!-style exclamations, "Cut the crap," (again, if the publisher/distributor allows the word crap) is one way to go. Other completely safe versions are, "Don't give me that," or, "Shovel it someplace else!" (Space can be a problem with these, but your imagination may come up with words or phrases I haven't though of.)

The point is that a lot of translators get stuck in their personal vocabulary. A good jog for the imagination are Monty Python skits such as the "abuse" portion of the argument-clinic sketch and the end of the abattoir-as-a-block-of-flats sketch. Neither of those use actual swear words (if I remember right), but some of the word choices they do use are pretty devastating. I'd love to use, "Malodorous," in a translation some time, but I haven't found the right situation yet.

Of course, when a translation calls for it, such as when one is working on a mature-rated title set in the mean streets, going the other way and using the nastiest language available to you is probably the way to go. One doesn't get that opportunity very often though.