I'm writing a prologue from the POV of a non-English-speaking character. How should I write the dialogue?

I've decided to use Italics and let the readers have the aha moment later on, as @Ángel put it.

Thanks for all the amazing feedback everyone!

Why bother? Shakespeare doesn't have Julius Caesar speaking Latin.

If there's something that's important to the story that doesn't translate exactly (a k'wel@p is similar to a horse, but with six legs) add footnotes or parenthetical comments.

Many of the previous answers are about dialog or solutions that would only work for a few lines, rather than a whole chapter or prologue.

The exceptions are:

• The "translated document" approach: Popular and works well unless you want your prologue to have immediacy, e.g., the POV character is involved in an action- or emotion-heavy experience that would not have the same impact in a letter or diary.
• Just use English / Let the reader figure it out: The best approach if you need immediacy - action/emotion

The latter is the best approach if the prologue is 1st person POV for a good reason.

However, if you do go this route, the English has to sound natural, like a native speaker.

Also be sure to give the reader some clues - given names and place names if your other race looks human as these will usually not be "translated" later, or if they are not physically similar to humans, you can also use descriptions of the people or surroundings around your POV character that show clearly they are not human without being jarring/out of place - e.g., a person having a healthy green glow or clacking their mandibles in irritation or juggling bags in their upper arms while holding on to their children with their lower hands/palps or their gills turning green from the disgusting taste/smell in the water, etc.

Think of the things you would see on a human face - flaring nostrils, frown lines, raised eyebrow/s, eyes narrowing, smiles, a nervous twitch, etc. and give your aliens some different body language along with the verbal.

When you don't have too much dialogue, you can use indirect speech to paraphrase the speech of your characters in English.

Julio asked Bella in Spanish where he might find a convenience store. Bella replied in Italian that he just had to walk down the road and turn left. Hans remarked in German that he finds it fascinating how Spaniards and Italians can understand each others languages, even when they can't speak them.

This has been touched on in other answers and comments, but I'd like to bring it out explicitly:

You don't have to do anything.

In fact, I'd argue you shouldn't do anything.

It's a general understanding when reading English books that in any given scene, "English" is the stand-in for "language the POV character speaks and understands". There are books - set in foreign countries or fantasy worlds - where there are zero English speakers at all, and yet all the dialogue remains in English. Nobody blinks at this, and you don't have to draw attention to it. Same for your situation: if we meet the prologue character in his setting as a POV character, there's no need to justify him not being able to communicate with another POV character later. Especially if the readers learned or could infer that his setting is one where English isn't in use. Language barriers! They happen!

(At the most, I'd suggest adding contextual clues if they're not already present in the prologue that we're not in an English-language setting. But these don't have to be excessive - if your character is called Vladimir Mikhailov and he's chatting with a Tatyana about when the flight from Yekaterinburg is going to get in, that is going to be more than enough for most readers. In fact, just the name alone is probably enough.)

On the flip side, explicitly drawing attention to the fact that they're not speaking English can violate your character's POV and alienate your readers from them. "He spoke in Russian" - this is fine if he's code-switching, but if the character is talking with other Russians, in a Russian-language setting, why would he even think that? How often do you go about thinking, "aha, I am going to speak to my friend in English now" if you're monolingual? Similarly, the suggestions about different typography, or even different grammar. Whenever you do this, you are basically forcing the story to say there is something special and unusual about something your POV character finds mundane (speaking his language). That doesn't mesh well and can lead to your readers distancing themselves from the character since you're making it clear that - unlike the English speakers - there is something weird about his perceptions. As someone who's multilingual myself, I have to admit seeing it in fiction bugs me every time.

TL;DR - just write the prologue dialogue in English. Your readers will figure it out.

I think you're making things hard on yourself. Is the book intended for people who speak English? Then the dialog should be in English. Readers routinely understand and accept that this is supposed to be a translation of whatever language the character would really speak.

Every now and then, I read a story where the author puts in a lot of text in a foreign language -- whether a real language, for example, if the story is set in another country, or a made-up language, like in a fantasy or science fiction story. Then they give the translation. This is just tedious. If I don't know the language, printing a bunch of text in another language is just a waste of time. I'm not going to study it. I'm not even going to read it. I'm just going to skip over it. If a reader does know the language, that makes it even worse, because know he's going to do his own translation in his head, and different readers will have a different experience of the story.

Books very regularly say things like, "Ivan and Svetlana spoke to each other in Russian. 'Did you find what you were looking for?' Ivan asked." Etc. That is, they say that the characters spoke in another language, but then they just give the English translation. No one finds this weird at all.

Most readers understand that foreigners, beings from other planets, etc., do not speak English, and you are giving a translation. Often it's not necessary to even mention this. If there's a point in the story where there's a language barrier, then yeah, you have to make this clear. But even then, it's no problem to the reader for you to write something like, "Francois turned to Emile and said in French, 'Let's speak in French so the Americans won't understand us." The reader is not going to say, "But why did he speak in English when he just said he was going to speak in French?" They get that it's a translation.

I want to suggest two other approaches, in addition to those mentioned by Sciborg.

One is just to establish clearly the setting of the prologue. If Yevgeny and Olga are having a conversation in a Moscow coffeehouse, the reader will infer that the conversation is in Russian and won't assume the speakers even know English at all. Later, when Yevgeny shows up in the main part of the story, you may have to make it clear whether he speaks English or not, but at that point the language of communication has some relevance. I honestly think this is the most common approach. Don't tell the readers things they don't need to know.

Another is to put the dialogue in italics. This foregrounds the fact that the language being used is foreign to the reader. I have seen this in science fiction stories, where there was a reason to emphasize the alienness of the context. Also, and I'm not sure about this, but it may be more common for younger readers, who have less experience figuring out that sort of detail from the context. But I don't want to demean this approach. It all depends on how much you want the reader to be aware, in the moment, that the language is different.

I don't recommend experimenting with different fonts in the manner of Terry Pratchett, as was suggested in the comments. That is a signature move of his and people will think you are copying him.

Having heard what you should do, allow me to mention something that you should not do:

If the character is speaking in his or her native language, and you are translating it into English for the reader's sake, then do not try to write English with the accent of the original language. Instead, write English that has the same sound/gravitas to English-speaking ears as the original speech would have to a native speaker of that language.

However, if the original speech involves slang, then the English rendering can (and should) use English slang to make the informality of the original speech plain.

It always struck me as odd when, for instance, two German officers in a WWII film, when speaking to each other, speak in German-accented English. The film The Death of Stalin handles this differently; although the different characters all spoke Russian in their day-to-day activities, the film is an English-language film, the actors are all British, and the variety of regional accents within Britain is used to represent the variety within the Russian language community.

The "subtitle problem" is an extremely common one in stories with multilingual characters, and there are a few different approaches. Here are three suggestions for how you can do it.

In most cases, you would do something like,

"What's the deal?" I asked, sliding easily into my native tongue. "Did the package come or not?"

This kind of transition immediately clues the reader in that the rest of the conversation is taking place in a non-English language without any more effort needed.

However, it might come across as slightly awkward if you are going for a more natural, POV-focused approach where you don't want the character to seem like she's actively thinking about such an obvious fact. In that case, you could do a transition like this:

Juana doesn't like talking in English, so I switch to Spanish. "Everything okay?"

In this case, it makes sense for the POV character to consciously think about why she's choosing to speak in the language because there's an in-story reason for it.

A third option is that if you, the author, speak the language fluently, you can outright have one character in the conversation speak the non-English language, and the other speaks English, where the responses to the non-English speaker will allow the reader to infer what was said through context.

"Que pasa, mijo? asks my father. "Everything's fine," I reply.

It's fairly clear that the father is asking how he is, and there's no need to spell it out.