How to include a foreign language in your novel?

So long as I know they're speaking in French you gain nothing by typing it in French.

More important is do the people to whom they are talking know what they're saying?

There's a variety of ways to do this, but generally the best way(s) ought to be where the reader knows they're supposed to be speaking a foreign language, but can still understand what's being said.

This may also be a good place to keep "Show don't tell" in mind. if you've managed to put an English word/ name to something and they announce themselves in French, the author gets the idea, but if you're saying everything twice, it might be kind of intrusive.

I've been working on a part in my novel where an English man has a conversation in Japanese for the sake of the guy he's talking to. At first, I just used English & only random Japanese words where I could get away with doing so by defining the term in the back & forth dialogue itself via the other characters' response. However when they both switched to full on Japanese, I just italicized & wrote the dialogue in the general manner of how Japanese is normally translated.

If it's just one line, I'd put it in French, then have the text around it clarify the meaning inside of the story. One of the factors that determines your treatment of the line is whether or not your main character, or the person whose POV you're using, can speak French.

If they can't, you can go toward:
***Police line in French***
Bob could only pick out the words for hands and door, but he got the idea. He needed to come out with his hands up.

Or if they can speak French:
***Police line in French***
I came out with my hands up, as instructed.

I mention the language and then write the foreign dialogue (in English) in italics just so the reader knows that that section isn't spoken in English. Whether or not most editors/agents are okay with that, I don't know. I got the idea from a few books that I've read in the past and it seemed like a good system.

If the police are banging on the door of a building that has been intruded in, I think that context suffices to tell the reader the general sense of what they're saying, even if the actual words are a language the reader doesn't know. I would hardly expect them to shout "Bienvenue a Prevost! Voulez-vous une tasse de thé et un livre?!" Given that, I would put it in French.

Assuming your character can understand French, I think it's best to use all English. Having a few words in French might give it the sense that another language is being spoken for the rest of the sentence, but to what end? It doesn't clarify the meaning, and in fact, confuses it.

If you're striving for realism, then the entire sentence would be in French (as you say) not just a few words. So I think that having a few French words gives you the worst of both worlds as it's not realistic, and not easily understood by the reader.

If your character can understand English and French words interchangeably, then his comprehension of what's being spoken is what you're ultimately trying to convey. Using one language preserves the flow of words as your character understands them without forcing your reader to guess at the meaning according to context.

I used some Spanish in my book. Just a smattering mixed with English. My publisher had me put all the Spanish in italics.

I don't mind coming across other languages in books! I think readers can generally figure things out, through what's happening, and what other characters are saying in reply. Context will fill in most of the blanks.

My first novel took place in 1880s Paris. I occasionally used a French word or term, but I did not sprinkle in "au revoirs" and French phrases into conversation, because it's generally frowned upon to use non-essential foreign phrases just for flavor. You are, after all, translating the entire novel into English from whatever language the MC speaks. But when the police show up, I occasionally refer to them as gendarmes. And any English words that have French origins, I use the French spellings and accent marks to make it more authentic.

You are supposed to italicize foreign words, yes, but only the first time the word is used. If the same word is sprinkled throughout the novel, there's no need to set it apart with italics again. And always make sure the meaning of the foreign word is clear, usually by the context.

Entire foreign phrases are, in my experience, frowned upon, so I'm not really sure whether you italicize the whole phrase or not.

One thing that I always wonder about, is how to deal with a language's grammar, when it's surrounded by English.

Say I'm writing a historical piece to do with Vikings, and one of the characters says "To Valhalla!" The Norse name for Valhalla is "Valhöll", but in this particular sentence, it would be "Valhallar".
That's a comparatively minor difference, and I imagine most people would notice it, but it wouldn't cause confusion. There are examples where it probably would, though. If, for some reason, I went through the whole book referring to ashes as "Aska", and then had to change it to "ösku", people would balk at it (and rightly so).

Easier to say "Bugrit", I tells ya.

Most of my WIP takes place in France so I have encountered the same issue. Most people I talk to say they would prefer to have it written in English and indicate what language is spoken in dialogue tags. My college English professors I still correspond with favor this as well. I think it's cumbersome so I wouldn't mind a critique of the example below.

My protagonist has been captured by a militia group. The group's leader has just walked into a room and sits down at a table across from the protagonist. Before anyone asks why the French man insists on English, there's a good reason that is revealed later. The following is the conversation:

"May I ask—" Long said in French.
"You may not ask," the man said. He scribbled something in the notebook. "I am Commandant Le Goff, Isére district, Front Républicain. I will ask the questions here."
The guns and the car would be hard enough to explain to the police, but talking to this outfit seemed a whole new can of worms. Long figured they had his fake passport, but he wasn't sure what story Sophie would tell. He decided to stick with the Canadian identity.
"Tell me," Le Goff said in English. "What is your name?"
"Adam Thompson," Long said.
"Where do you live?"
"1526 Forest Bluff Crescent, Mississauga, Ontario."
"What is your relation to the madame traveling with you?"
"We are professional associates."
"English, please," Le Goff said. Long repeated his answer in English.

Two great articles related to this subject:

How People Really Talk: Language and Signaling Difference (watch the referenced video too; it's well worth it and quite funny)
Stand Back! I’m Going To Quote Junot Díaz (Thinking about language)

Highly recommend them both!

I have just done a few scenes with exchanges in Chinese. As neither I nor the pov character understood what was being said, there was little to be gained from showing as opposed to telling.

slhuang said:

Two great articles related to this subject:

How People Really Talk: Language and Signaling Difference (watch the referenced video too; it's well worth it and quite funny)
Stand Back! I’m Going To Quote Junot Díaz (Thinking about language)

Highly recommend them both!

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Interesting articles. Does make me think twice whether I want to include the other languages in italics or not.

I am currently reading Dan Brown's Inferno and love the way he's written the Italian dialogue within his pages. Not that this is the only way, but I am enjoying learning a little Italian as I read it and it flows well. If the language matters to your story, go ahead and tell it in the French.

Personally, I would have no problem with the entire sentence being in french if it fits better in the story that way. I've never dealt with this in my novel or short-story writing, but I have used vulcan or elvish in my fanfiction. Depending on the context a phrase, sentence, or specific word being spoken in a language other than english makes sense. I never do it if it doesn't add anything to the story though, and if it doesn't, chances are whatever the character was going to say should be changed anyway.

It can be done and done well. Just make sure it is really the way you want to go.