For those of us who write fanfiction based on anime and manga, there's a continual task of deciding how to convert Japanese words and concepts into English, even if we're not dealing with a direct source. I use the word converting here because, while not all of us can read Japanese or make sense of it (I know I can't), the concepts still exist and, if they're foreign enough to the culture we come from, they demand to be presented in a fashion that makes sense.
The history of dubs and translations (whether official or otherwise) has been one of increasing tolerance and presentation of Japanese concepts as they are, rather than trying to force them to fit more familiar Western constructs. More and more these days, manga is sold in the original right-to-left format, honorifics are largely left intact, and so on. In part, I think as anime and manga have become more mainstream (I won't say that they are mainstream, but I think there's a much greater cultural awareness of them than in previous decades), publishers and distributors have been more willing to leave these subtle details--these aspects of Japanese culture--intact. I've said before that writing a piece of fiction means that anything you do, any choice you make, has a statistical chance to turn people off and walk away. I think the same applies to that market (or at least, the belief that it does holds), and it's only when publishers could reasonably think most customers won't be deterred would they do it. It's not about art--at least, not all the way. Money matters, and just like natural selection leaves only the organisms and parasites fit enough to survive, the marketplace leaves only those companies and organizations that are fit enough to sustain their operations, to profit.
Often, translations are caught between capturing the literal sense of what is said and the sense of it instead. Puns are especially difficult--it not almost always impossible--to translate and still carry the humor they would've had in the original language. The same goes for aphorisms and other sayings. Translators have approached this problem in different ways, and you might even say the best approach still depends on audience and context. Fans are more likely to know some of the cultural tidbits that might be present in anime and manga and thus we can handle them being presented intact, but not everyone is like that, hence the effectiveness of the Woolseyism in some contexts.
It's the sense of things, the connotations of words, that I want to focus on more, though. An author must understand how the connotations of words--the ideas and feelings a word conveys aside from what it literally means--affect the overall depiction of what's being portrayed. It means different things to call someone a bold explorer versus a brash adventurer. The latter brings about images of rash, reckless behavior while the former only speaks of courage in the face of adversity. In this way, writing is like painting, and the color and tone of the words we use is like subtle shades of pigment on a canvass. It can change the feel of a work from positive and exuberant to dark and melancholy. It can make a character look like a brave hero or a foolhardy meddler.
This all goes double for dialogue, where you not only have to worry about the connotations of words but sentence structure and vocabulary. That is, what is a character likely to know? How do they speak? Do they call the call the car that just passed by a convertible or a corvette? Do they call it red or crimson? Is the storm coming in making it windy or blustery? The distinction between plain words and fancy words is usually enough to distinguish someone of "education" or high upbringing or what-have-you. Even if you can't translate the Japanese way of honorifics or polite verb conjugations, you can have someone speak formally even in English dialogue and get the idea across. For the Ranma fans out there, Kodachi is a good example. She's a little nuts, and someone like Ranma might have no qualms saying so to her face. How would she respond? "I'm not 'nuts'! I've been to eight doctors, and they've all told me I'm fine!" Or more like, "I assure you I'm perfectly healthy. Several physicians have assured me that my faculties are sharp and intact." Even the exclamation points make a difference.
Now, what motivated this post was just that: the writing of dialogue and how it reflects on the person saying it. In particular, the differences between commercial translations of a source and scanlations, which can of course be of varying quality (both of them). For the moment, I'll confine the discussion to Mahou Sensei Negima, volume 4, chapter 33.
To begin, page 6, panel 1 (top right):

Left: Del Rey version; Right: version currently on Mangafox. I apologize for the crappy scan of the Del Rey version. Clearly the art of scanning manga isn't done by placing the book on an HP scanner and hoping the spine doesn't get in the way, but I digress. This is a case where the intended meaning that comes across is virtually identical, but the construction of the sentence, in my mind, gives the two versions subtly different flavor. In my mind, the Del Rey version comes off more boldly yet also more negative--as if Miyazaki, the character in the panel, expects to be rejected. The scanlation here seems more neutral, but already it seems like a valid question: is this a girl who has a negative conception of herself, or is she merely shy and deferential?
Admittedly, I think the proper interpretation is a little of both, and the different versions simply emphasize it differently. Even so, I think the slight discrepancy in characterization is important. I think what we have next is a much more clear-cut case:
Page 14, panels 5 and 6 - Characters shown: Miyazaki (far right), Kagurazaka (next to her), Sakurazaki (left and leftmost panel)
Scanlation:

Del Rey:

Without pulling any punches, the discrepancy here really bothers me. This is, as I described above, the big difference between formal and informal speech, between an ordinary 15-year-old girl and the very aloof, very stiff character that Sakurazaki is. She's stoic. She's a little strange (well, maybe not so strange if you bring out Negima standards, but she's surely not meant to be normal). Is it a big deal to have her say "no biggie" in response to something? Is it that much worse than "it's ok" or common variations of that? Maybe not. Maybe it doesn't matter that she refers to Springfield as a kid rather than a child, but like I've said, this stuff adds up. And perhaps this was done intentionally, to make these people sound more colloquial and, thus, make them feel more familiar. That's a possibility. But I also think that people have a much greater tolerance for unfamiliar cultures in literature and art than we give them credit. What about Lord of the Rings is immediately familiar to the modern reader? Don't we still accept the vast and myriad ways of the people of Middle-Earth regardless?
It's hard, in this context, to show thoroughly all the things I might nitpick between these two translations, but this is something that broadly applies to fiction--fan-made or original, translated or native English. The words we use matter, and choosing the right word for the person, for the situation, gives an author a great chance to convey complexity and feel without having to outright tell the reader.
And really...

There's just this little nagging part of me that says, "This girl's carrying a sword--why is she even using the word ditching?" Yes, even if it does mirror what Kagurazaka says. You might say it just bugs me.