Untranslatable
Sensei's very first Japanese sensei had a saying, "No translation is no translation." Basically what it meant is that it is the translator's job to put the Japanese concept into English, and if the translator leaves words in Japanese, the translator is not doing what he or she is being paid to do.
That's one extreme of the positions on this idea, but there is a valid point behind it. Human language is made up of concepts that all humans share. There may be points that are relatively more important to one culture than to another (like the Inuit's number of words for snow that everyone has heard of), but there is no concept that one person can think of in one language that a different person can't think of in a different language. Actually, that's the basis for all translation -- that the human mind created language, and if something can be said in one language, it can be said in another.
There is, of course, the opposite extreme. That what is said in a language comes along with nuance and cultural baggage such that no translation can ever be wholly accurate. This is also a valid point. If I said that a couple of celebrities were, "acting like the second coming of Bennifer," then I have just referenced in your mind a few years worth of celebrity gossip that everyone in North America was exposed to. A culture that didn't follow American celebrities would just say, "Huh?" And it doesn't have to be so obvious. Words like, "Y'all" carry a certain cultural baggage that would be very difficult to translate, and even non-dialectical phrases like "Military Institute for Boys" carry certain cultural meanings that the words themselves don't carry. Taking this to a further degree, one could argue that since the culture helps along in the background when understanding a language, no language can hope to be translated and give the same feeling to a reader in another language.
Obviously I don't subscribe to this second extreme, because if I did, I'd hang up my dictionaries in the closet and leave the profession. No, what we translators are working with is somewhere much closer to the first extreme than the second, but there are elements of the second in our translations too.
The fact is most anime and manga fans want a little Japanese in their translation. Although people who buy into the first extreme (and I was one of them for a while) feel that anything that can be expressed in one language can be expressed in another -- it might just take a few extra words to do it -- the truth is we are ultimately paid by our readers forking over their cash to pick up our translations.
I'll tell you where I became a convert to the having-some-Japanese-in-my-translation-isn't-such-a-bad-thing camp. I was editing Fushigi Yûgi for Viz, and someone wrote in about the character Chichiri who ends every sentence with the Japanese sentence-ending particle "no da." (To be technical, "no" is the particle, and "da" is the copula.) It was running in the magazine Animerica Extra which has a letters page, so I ran the letter, and as a response, I said, "I don't know what we're going to do. What do you think?" (Actually, at the time, I was still a convert to the must-translate-everything school, and I was leaning toward leaving the "no da" out.)
It was there that I learned one cardinal rule for working on a letters page. Don't ask a question you don't want to know the answer to. I meant the, "What do you think," as a prompt for people to send in letters to the magazine, but many fans took it as a poll. They put up on websites that I would allow the fans to decide whether Chichiri leaves his "no da" as is or not, and I suddenly got flooded with mail. More than a hundred e-mails a day. Most of them were just writing in without even knowing what they were writing in about. The majority wrote in saying variations of, "you better put no da in teh anima or i never watch!!!!!!!" In other words, people who simply clicked on the link and wrote what they thought the website owner wanted them to say. There were many others who did read the book that used threats such as organizing a boycott of the magazine or coming up with a petition. None of this did anything to change my mind. (It's very easy to become stubborn when confronted by threats.)
Then one fan wrote in saying that she would be made very happy by leaving in the "no da," and it brought me up short. Really. Suddenly I was confronted with a point that I couldn't argue. Making the fans happy is a good way to increase sales. I still feel that if a line of dialog can be translated, it should, but that doesn't mean that a little bit of easy Japanese thrown into the mix is a bad thing. The Japanese do it when there are English-speaking characters in the manga. The foreigner will usually say "Hello," in English, then go on to speak the rest of the dialog line in Japanese. (Although in real life, the only Japanese most foreigners know are the greetings, so the dialog would be the other way around. "Konnichi ha" in Japanese and the rest of the sentence in English.)
Of course the company I do translations for (Del Rey, may their manga ever boom) requires a certain amount of Japanese (honorifics, etc.), so the decision is made for me, but after the incident with Chichiri's "no da," any objections I once had have been quelled.