Perhaps it can be said that there isn’t a single aspect of any language that is quite as… colorful…as slang and, more interestingly, dialect. Where many languages can often be linked to each other in terms of grammar, vocabulary and structure, slang and dialect are where each language sets itself apart, and lends a distinct “flavor” to the words spoken.
Without going into too much detail (wouldn’t want to attract the wrong kind of crowd through Internet search engines ;) ), blasphemy and references to genitalia are very common in most forms of slang, but Dutch for instance (one of the most creative languages when it comes to slang if you ask me) uses lots of diseases on top of that, and Japanese has such a peculiar way of expressing contempt, that I’m still not sure how it works (one of the most insulting words, Kisama, consists of an honorific prefix paired with an honorific suffix for cryin’ out loud).
In that sense though, translating slang isn’t all that hard, because you have a specific vocabulary to work with. Dialect, on the other hand, poses a far trickier problem from a translational point of view. Each dialect is likely to have a certain stereotype attached to it, which could prove very hard to localize if the target language does not have any dialect with the same kind of image.
One of the biggest dialects in Japanese is Kansaiben, spoken in the Kansai area surrounding Osaka and Kyoto. This is a dialect that, despite being quite different from Standard Japanese (which is more associated with the Tokyo area) has found its steady place in popular culture, on TV, in comics and in videogames etc, which is probably largely due to the fact that it is associated with comedy (Yoshimoto Kogyo, one of the largest entertainment conglomerates in Japan was founded in Osaka). Furthermore, because of Osaka’s background as a merchant city, it is also often associated with commerce.
The problem is, how do you translate this to English (or any other language for that matter)? Most languages tend to have at least one dialect that is associated with the countryside (and by extension “backwardness”) and so does not pose too much of a problem when translating, but when it comes to dialects with a very specific image, like Kansaiben, what do you do!?
Often, especially in written texts, it is not translated at all, losing some of the charm of the original text. One popular solution in the American localization of many Anime seems to be to include characters with an Australian or British accent, which can be used for comic effect, but still have entirely different associations. On the other hand, in the Japanese dubs of foreign movies/cartoons/games, dialect is often ignored despite having a wealth to choose from (one famous example of the contrary is Shrek, whose Scottish accent was replaced with Kansai dialect by a popular comedian from Osaka).
It would definitely be nice if translators would make the effort to create translations that are more diverse, not only for comic effect, but for the sake of believability as well.