What's the best way to show a character speaking a foreign language in fiction manuscript? Should the foreign words be italicized and include a translation? Should it just be included in the sentence and try and make sure the reader can guess at the meaning by using the surrounding sentences and words? Is there another way that's not coming to mind?
You have two options depending on context:
1) If it's a quick exchange and can be figured out in context, put the foreign language in italics.
"As-tu le livre?"
"Yes, I have the book here."
"You filthy p'taQ!" B'Elanna snarled.
2) If it's a quick exchange without context, put the translation afterwards and italicize that.
"Pour ma peine, ma punition, je tourne en rond," he sighed. For my pain, for my punishment, I pace in circles. Now Picard understood.
"Qa'pla!" Successs! the Klingon shouted.
In any case, I would not have more than one or two exchanges in a foreign language. Either use a tag like "she said in French" so the reader realizes the characters aren't speaking English, or note in narration "they discussed the matter in French for some time, but as Malcolm didn't speak the language, he had to wait for a translation."
The way I see it, if the foreign language usage is important to the story, then use it in italics. If not, just avoid putting it explicitly in the text.
For example, assume you write a fantasy novel in which Elves always add the word Ur-Sook when addressing little children.
If you just wanted to make sure the reader understand that the Elf is busy, the second and third examples will do.
However, usage of the the first example can help you develop your story.
The reader will probably wonder what Ur-Sook means, and maybe guess this is the child's Elvish name. But then you can twist it around by having a mature Elf use that word when addressing another mature Elf, which will lead to a fight among the two...
In this case. the usage of the word and its inclusion in the text is important, so the first example is more appropriate.
Try to avoid using another foreign language as a stand-in for the language you're wanting to portray (like, say, using Swedish as a stand-in for Romani, as was done in Thinner). I'd treat that as the most absolute requirement.
Try to avoid long passages in another language. If you're finding yourself using much longer sections than "a sentence", it will probably be too intrusive.
For single sentences or shorter, provide something along the lines of a translation. This should probably be to the level of understanding of the scene's viewpoint character.
FWIW, he's shouting "rot like a banana", pretty silly as an interjection, but at least not very offensive.
It is possible to use multiple languages even if you actually don't by flagging them in dialogue tags. This is good when who speaks what language(s) is important in the narrative. You can put the inter-language confusion in the dialogue and the speakers' actions. You can also use this to play with mis-translation, either deliberate or incidental.
If you know what you're doing, you can also use idioms differently to subtly indicate a different langauge. For example, in one of my fantasy worlds, I characterise one of my languages by limiting contractions and by repeating verbs in a way that is not usually done in casual English.
This technique as also a good out if the languages are fictional, or if you, the author, don't actually speak one of them.