This is a collection of stuff I’ve already said elsewhere or on this blog, but for what it’s worth… The usual disclaimer applies: these are my personal opinions and my personal experience (I know not everyone has the same opinions and I certainly don’t pretend to speak for everyone!). I also don’t pretend to have easy solutions for everything I mention here (and God knows I made some of those mistakes myself, and will continue making them, but hopefully I’ll improve on that front as time goes by); but I think it’s better to know all this stuff and then decide how to handle it rather than go on being blissfully unaware of it.
Warning: this is me in ranty mode, not helped by the 3 hours of sleep I got over the past few days (yup, I know that I volunteered for that whole sleepless thing. But doesn’t change much to how I feel…)
1. Researching another culture is freaking hard work, PLEASE do not undertake it lightly (and when I say “freaking hard work”, I don’t mean a few days on Wikipedia, or even a few days of reading secondary sources at the library). And PLEASE do not think you’ll be exempt of prejudice/dominant culture perceptions/etc. No one is.
2. Read your sources; read primary if possible. Reading primary and/or sources written by people from inside the actual culture is very important, because there’s a boatload of really faily outsider books out there that are still held up as examples of accuracy. Not saying you shouldn’t read books by outsiders (it’s also important to have reference points you can use if you’re not from the culture, because some things are so obvious that insider books won’t stop to mention them at all), but you have to be very careful and very critical of what’s inside them. A lot of the stuff I read about Vietnam just feels hilariously off to me; and I’m a second-gen, not someone actually living in Vietnam, who presumably would find even more hilarious bits.
Try–inasmuch as possible–to be aware of your prejudices when reading. You can’t hope to view 13th-Century China by applying your 21st Century (Western or otherwise) values to it: the fact that women were “oppressed” by modern standards doesn’t mean they would have described themselves in such a way, nor does it invalidate the entire civilization (and nor does it mean that sexism and/or masculinity took on the same forms as they do now, or as they did in Ancient European countries). Note that this isn’t me advocating moral relativism; this is simply that you can’t hope to write something set in another culture if you do not understand what makes that culture tick (or you’re writing propaganda against that culture, in which case you probably don’t need me…)
3. Be aware that you’ll never write insider narrative, because you’re not an insider. That in itself is not necessarily bad, but be aware of two things. The first and most important point is that outsider narratives have a tendency to string together bloody offensive clichés (generally the perception of the dominant culture you grew up in) and are totally oblivious to that fact. I have lost count of how many narratives on China  featured any combination of the following: over-formality between members of the same family (because everyone knows that Chinese is a formal language! Guess what. Most communications within the family are brutally simple, because the respect is already implicit in the relationship itself); use of broken English (because all immigrants/foreigners speak bad English!); reference to women being tiny and fragile and exotic, or a combination of all three (a cliché that might not be a problem; but if you’re in a Chinese-dominant universe, where–guess what–everyone is equally small and “exotic”, it is most certainly totally unjustified); everybody or nearly everybody being experts at martial arts (not to say martial arts didn’t exist, but you have to be aware that they certainly weren’t mainstream, and that a Confucian court official is unlikely to be an expert in them–more like really suspicious of those dubious sporty techniques since Confucians hated sport). And the list goes on…
If you get past the stage of clichés (and a vast majority of outsider narratives don’t, so do take some time to think on what it is that you’re writing and how you’re presenting the culture), the most frequent and insidious problem of outsider narratives is tone-deafness, aka putting the emphasis on what seems shiny to you (and totally commonplace to insiders), and/or casually using important, traumatic details with no idea of their importance. For instance, referring to people or features of the Vietnamese/American War as an easy way to set the background for your story? Those are NOT casual mentions; they’re linked to events that took place barely a generation ago; and they’d be pretty traumatic for most Vietnamese. Similarly, if you geek out on durian in your food descriptions, that’s a bit like your French characters geeking out on strawberries–sure, if you’re a foodie (and even then, they’re unlikely to describe strawberries in loving detail, but will rather focus on what makes those strawberries so extraordinary compared to the standard ones). That’s just the small stuff–there’s also the big stuff such as cultures simply not having the same emphasis and the same values as the one you come from (being a scholar in Ancient Vietnam? The most prestigious and famed occupation, the dream of all mothers for their sons. Being an academic in 21st-Century France? Opinions will vary, but there’s certainly not 90% of the population for whom this is a dream job).
4. Drowning out: apropos of outsider and insider writing, if you’re not from the culture (and especially if you’re from a more dominant majority), be aware that your narration will be that of the privileged (whether you’re the most privileged strata of your society or not), and that as such it has a strong likelihood of being taken *more* seriously than actual work by people from the actual culture. This is problematic on two levels: one is that, as said above, outsider narratives can give a more or less false image of a given culture (and thus promote problematic representations, again on a more or less serious level, reinforcing the majority perception of that culture); and, second, because, for good or evil, works set in an “exotic” culture are perceived as part of a limited market (ie, if your work features, say, Aztecs, the perception is that there aren’t many slots for Aztec novels, because those “are all the same”. I know no one says that of works set in the US and featuring straight white men, and that this is an unfair perception, but it doesn’t change that it exists). And because the market is limited, that means that publishing that kind of work removes space for insider narratives to exist (again, in the actual state of publishing. Change is coming on that front, but like all changes it takes time). Again, not discouraging you from writing what you want to write (I’d be the last one in a position to do so!); but it’s good to ask yourself why you’re writing what you’re writing; to be aware of the consequences; and to promote writings by people from the actual culture in addition to your own–because they have voices of their own, but more trouble getting heard.
5. If you find yourself twisting the research to fit your original plot idea, ask yourself if you’re really going the right way about it. For instance, if you wanted A in your plot set in Ancient Vietnam, and it turns out Ancient Vietnam has no such thing as A, for the love of God please don’t go about grabbing obscure parts of Vietnamese culture and twisting them so you can still have A (or, if you must do it, please stop pretending right here, right now, that you’re being respectful and that you did your research). Take a long hard look at your premise which includes A, and ask yourself if you can’t tweak or remove A from it. That way you won’t find yourself having Vietnamese dragons who eat people for lunch because your plot needed it .
6. Using readers from inside the culture: it helps *a lot*. But be aware that they’re not omniscient; and also that they might just be too polite to give you honest or strong feedback. If they raise points in their critique that they self-label as small things… be very, very careful of dismissing those as actual small things. This might just be their way of telling you you’ve screwed up big time. Also, quite obviously, you can’t go around saying “my narrative is perfect because one person from culture XX thought so!”.
7. Using foreign languages in your narration, aka my personal pet peeve (sorry). I tend to think that this is like using edible glitter in coffee ; easy shiny exoticism and a quick way of saying, “look, we’re not in the UK/US/France anymore”. If you think about it, though–the entire narration is in English, presumably “translated” from whatever language your characters really think in. Why single out a few words for special treatment? You can argue there’s no equivalent in English; but most foreign words don’t have equivalents in English (and very often, it’s not those “difficult” words that get dumped in the narration, but simply a random scattering of words). Also, if you don’t speak the language (and by “speaking”, I mean “almost bilingual”), you simply run strong chances of not having the right words, because they wouldn’t be used in this context, because they’re the wrong diction level, because they’re acutely wrong for your time period. One book I read used “salaud” as an insult in Medieval France, which really threw me out of the narration since “salaud” is quite a modern insult. Ironically, if the entire book had been written in modern French, I probably wouldn’t have batted an eyelid, but because there was no French context for this use of the word, it merely looked wrong.
What about using a native speaker to do your translation? Most of the time, the translation requests I see go something like this: “can someone tell me how to say ‘A’ in French/Mandarin/etc.?”. There’s no context, or insufficient context, and sometimes ‘A’ simply doesn’t exist in French, or has several very different translations depending on who’s speaking and what they’re saying. And the author, quite happily, takes the answers they’re given and run with it, unaware of the issues underlying the translation.
8. Bonus round: on diversity in SFF. Been following the hashtag only distantly; and I’m all in favor of more inclusivity in the field in general. However… Diversity in SFF is not *only* more diverse characters written by white authors (and “more diverse” doesn’t just mean funky skin colour and weird eye shapes, but a taking into account of cultural differences at bedrock level); but also, as said above, encouraging the expression of voices from other sources than the dominant culture (LGBT folks, POCs,…). And in at least one respect, what I’ve seen has been hugely frustrating, because there’s a huge assumption that people from other countries than the usual Western Anglophone suspects (US/UK/Can/Aus/NZ) cannot possibly write in English and that “international SF” is shorthand for “translated works”  . It’s a very… monolingual assumption (guess what, people all over the world have to learn English because it’s the dominant language, and we end up speaking it quite well, thanks, well enough to write stories in it and/or function quite normally with native English speakers); it’s even worse than that, because there’s a lot of non-Western countries where English is the *first* language, or at any rate an official language (India, Singapore…), and authors from those countries are also being erased. You can take a look at the output of people (mostly) writing in English over at the World SF blog.
(with thanks to @kiplet, Bradford Lyau, Kathy Sedia, and Kate Elliott)
EDIT: a few other posts on the subject, well worth reading:
 Again, I’m using China as an example because there aren’t so many Vietnamese narratives lying around. If any Chinese people are around and would like to correct me, feel free!
 Vietnamese dragons are Heavenly messengers, and humans aren’t really part of their diet. Meeting one is only dangerous insofar as meeting angels is dangerous–they’re beautiful and terrible and not part of the mortal world, but they’re really not going to savage you for no reason (unless, possibly, you’ve incurred Heaven’s wrath, in which case a stray dragon is really going to be the least of your problems).
 Sorry. I’m biased against glittery coffee. Though on, say, chocolate cake, sparkles do look fabulous!
Not that there isn’t a huge problem of unbalanced translations (from English to other languages vastly outweighs from other languages to English, and if you don’t write in English, your visibility in the English-speaking world is pretty weak, to say the least).
 With the attendant “worship” of translations–I’m not saying translations are an easy art (as someone who speaks several languages, I’m acutely aware that a good translation is an uphill climb requiring as much, if not more work than the original writing; and the translation of humorous stuff like Terry Pratchett’s Discworld into French, for instance, has definitely got me in undying awe). But the “translation is a difficult, awe-inspiring art” all too often seems to devolve into a more-or-less conscious reinforcement of the mystique of translation. At best, it simply deflects the conversation away from the dynamics of power of Western Anglophone publishing vs rest-of-the-world publishing to nuts-and-bolts questions and anecdotes; at worst, it serves as an underhanded justification that things don’t get translated into English is because translation is too hard and impossible to get right; and *that* is a patently false idea.