When fansubbers get cute
Character A hands something to Character B.
Character B (speaking): Oh, sankyuu.
Character B (subtitle): Oh, domo.
This brings up a good question, though. If you're writing fanfic in English about characters who speak Japanese, do you try to render the occasional bits of English they use in some special way to show that it's English and not Japanese? There doesn't seem to be a straightforward way to do that and I usually just don't try.
Character B (speaking): Oh, sankyuu.
Character B (subtitle): Oh, domo.
This brings up a good question, though. If you're writing fanfic in English about characters who speak Japanese, do you try to render the occasional bits of English they use in some special way to show that it's English and not Japanese? There doesn't seem to be a straightforward way to do that and I usually just don't try.

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I'm re-reading Vikram Seth's A Suitable Boy right now, which usually tells you whether the characters are speaking English or Hindi, when they switch, and how strong their accents are (especially whether or not they have Bengali accents). But these characters have full paragraphs of conversation in each language; they're not just inserting a foreign word here or there.
I suppose if I wanted to show a Japanese character used bits and pieces of vocabulary from other languages, I'd pick something besides English (e.g. French for someone showing off.) But I suppose straight-up "He said, in English" has its place:
--I love you.
--I love you, he said in English. He looked away.
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