Say two characters are having a conversation in their native tongue (which is not English), do I write it in English then explain how they were talking in Russian or whatever? Or do I write it in Russian? Then translate...? This may be a bit of a silly one, and a no-brainer...
I'd say write it in english but insert a few well chosen words in russian, I think people use to write that in italics but Im not sure if that is correct. Maybe you can also add the translation for that russian word right after, if it's not understood by the context. I write in swedish but since the story takes place in an english speaking country I write it as if it would already translated and ready,lol. But every now and then I put in a word just to get the feeling of what is being said. For example if it is a word which doesn't translate perfectly into swedish, usually those words we all know here anyway, since most people speak english at some level. Maybe you can have a look in another book to see how they have done it, then you decide what you like the most and what doesn't get confusing. Since not everyone is familiar with russian ( or whatever language it might be) you could either use words that we could know(in many languages there are words we all know), like "thanks" or "yes", and in that case I dont think you would have to translate them, or if its a more difficult word you write the meaning of it too. If it is really russian, it would be hard to write in that language since they have another alphabet anyway...
The best answer is just write it in English. Establish at the start of the conversation that they are speaking in Russian, then write in English. The reader will follow your thinking, especially since chances are he won't speak Russian. If the story takes place in Russia and the characters are Russian, you can even make the reader assume they speak Russian, like in historical dramas where French aristocrats or Mongol generals speak English: the audience knows they would have actually spoken other languages. Some writers write in the characters' tongues, then follow it up with the English translation in italics. This can get annoying but is an option. But unless you actually speak and write Russian, and well, don't bother: it takes too much time. Orson Scott Card occasionally has characters speak Portuguese in his books, and writes it in Portuguese--he can do so, because he speaks the language. But if you don't speak Russian don't write in Russian, it's too much work and there's too much room for error.
Generally you should write it in English, but there are times when you may want to write the actual foreign language. For instance, you may want to emphasize the fact that so and so character speaks a foreign language. For example, in one of my stories, one of the first scenes deals with a little boy meeting a little girl for the first time. The boy, who is Vietnamese, wrongly assumes the girl is Chinese, even though she is Vietnamese like him. When she starts spouting out random Vietnamese, he's utterly shocked. I wrote her Vietnamese in Vietnamese, because it was just a few words and I felt it was more important to get to the point that both of these characters speak and understand Vietnamese well.
We had a long discussion about this a while back, but basically - only write in a foreign language if you know it will be correct. Have it checked by a native speaker if you are not one yourself. Otherwise you end up with something that may be incredibly wrong and will put off anybody who knows the language.
That's only IF you're going to write in a foreign language, which most of the time an English speaking writer writing for English speaking audiences should not. But yeah, I agree, if you're going to do it, do it professionally. One interesting note is in A Scanner Darkly Philip K. Dick has whole sections of prose in different languages. He was actively trying to demonstrate the severe disconnect the character was experiencing that was making him insane. If that's the kind of effect you're going for, then writing in other languages is a great idea! Usually, like with accents and speech impediments, it's enough to reference the fact and indicate it so the reader understands, without needing to be cumbersome or confusing with translations (edit: or worse, a lack of translation, heh). If the character doesn't understand the language, then it might be a nice effect to write in the foreign language they don't understand, but also keep in mind you're now making a judgment and assumption that your reader won't understand that language either. If they do, then you've effectively broken from the pov/perspective of the character, so that tactic is limited. My advice is always the same: write the truth of a moment through the character's experience. In a moment where others are speaking in a language you don't understand, you aren't cataloging and documenting the language. You're internalizing what's going on, only idly hearing the speech, picking up words here and there that seem familiar and trigger a response, wondering what they're saying, etc. So, don't write the language, as that's not what matters. Write the pov character's experience in the scene/moment, as that's the only thing that does matter.
If the POV is from the MC, and they don't speak the language, that it might be good to include the foreign words, to keep the reader feeling what the character is. The reader knows what the character knows. But if omnicent then I would tend to go with English words explaining they are in another language. I have to go back and change some of my books, I used Tolken's elvish with translation for when the elves spoke to each other.
when it comes to foreign languages, keep in mind that a little goes a long way... it doesn't take much to annoy the readers, whichever technique you employ...
Bingo, it's a bit of a put off when an author drops some foreign words on a reader, sometimes just assuming they know them. Are we supposed to consult a foreign dictionary, just to make it to the next chapter? That can really throw off the flow of a story. I say use English if there are more than just a few words, try to explain it through storytelling. The two men spoke in Russian VS writing a few lines in the Cyrillic alphabet, which will really throw someone off. Unless someone is proficient in a language, attempting to write in one can be an artform unto itself. And even then, there are just certain sayings and idioms that may not translate well into English.
