I'm writing a fantasy novel and one of my characters speaks in English, but sometimes utters single words in an ancient dead language, and I don't want to abusively use that language.
For example:
"You're going to be dead SAHU²!"
Is using a footnote a good idea, since the words are not too many? Or is it preferable to write the translation in italic, like:
"You're going to be dead SAHU! pig."
Most of the answers given so far seem to be some variation of "do what the masters did". The trouble with that is that the "masters" were often writing at a time when French was the lingua franca, and when anyone with any education could be assumed to know Latin. Neither of those things are true any more, and I for one find it immensely frustrating to read classic stories by the "masters" with large amounts of untranslated French and Latin. I'm therefore not convinced that the works of the masters are necessarily a good basis for an answer to this particular question.
Instead, write with your readers in mind.
Whether the language you're writing is purely fictional or real (and common or obscure - in absolute terms plenty of people speak both English and Sanskrit, but proportionally, not many), if your reader can be expected to infer the meaning purely from context, it is reasonable to leave it untranslated. Otherwise, options are to provide more context, or to repeat in English inline, or to provide a footnote - whatever will be best for the reader.
Examples:
"You're going to be dead SAHU!"
This is fine. The reader can infer that SAHU is an insult or epithet. There's no real need to know that it means "pig", nor for a footnote that takes the reader away from the flow of the text.
"Allebeth farnor geruntin, blessings be upon you."
A reader would normally understand that the speaker may not be literally repeating themselves, but that they are speaking in a foreign (in this case, purely fictional) tongue with the translation provided.
"Allebeth farnor geruntin."
"Blessings upon you too, my friend."
Here, additional context is provided to allow the reader to understand the (fictional) language with no effort or confusion, and no need for a direct literal translation.
"As our old Hallenor teacher used to say, farva caner biot!1"
The context here suggests that the speaker and the listener would understand the (fictional) foreign language used - but the reader won't. In this case, it might feel awkward to provide the translation inline as the characters wouldn't need it, but a footnote is a perfectly reasonable solution to provide the reader with the information they need.
In each case, consider what is likely to be best for the reader, and use the most appropriate technique accordingly.
Some examples:
Raoden breathed a sigh of relief. "Whoever you are, I'm glad to see you. I was beginning to think everyone in here was either dying or insane."
"We can't be dying," the an responded with a snort. "We're already dead. Kolo?"
"Kolo." The foreign word was vaguely familiar, as was the man's strong accent.
(Brandon Sanderson, Elantris, chapter 1)
No footnotes, no inline translation. The fantasy-language word is understood from context.
'O Fair Folk! This is good fortune beyond my hope,' saod Pippin. Sam was speechless. 'I thank you indeed, Gildor Inglorion,' said Frodo bowing. 'Elen síla lúmenn' omentielvo, a star shines on the hour of our meeting,' he added in the high-elven speech.
(J.R.R. Tolkien, The Lord of the Rings, book 1, chapter 3 - Three is Company)
The translation is provided by the narrator, without breaking the flow of the narration.
When he saw Strider, he dismounted and ran to meet him calling out: Ai na vedui Dúnadan! Mae govannen! His speech and clear ringing voice left no doubt in their hearts: the rider was of the Elven-folk.
(ibid, chapter 12 - Flight to the Ford)
No translation, readers can only guess at the meaning, beyond the general "hello".
Those examples have a commonality: none use footnotes. Why? @Amadeus is exactly right on this - because a footnote takes the reader out of the flow, it breaks the immersion in the story. It makes the reader stop, and go look at something else. It's an interrupt in the thought process. A stumble in his journey. Footnotes may be used when this is the effect you want to achieve. For example, Sir Terry Pratchett made use of footnotes for comedic effect.
If the general meaning of the foreign word(s) can be understood from context, translating is redundant. There is therefore no need for translation in the Brandon Sanderson example. The example you provide is similar - the fact that 'sahu' is a curse can be inferred from context. The specific meaning thus becomes unnecessary.
If the general meaning cannot be inferred from context, does the POV character understand the meaning? If he doesn't, we probably shouldn't either. If he does, and if the story has something to gain from the translation, the first Tolkien example shows how a translation can be added without breaking the flow of the narration. In the second example, we do not understand the phrase, but nor do we need to. It's a conversation we're not meant to intrude on, only see that it is happening. (Also, it contains a spoiler.)
Footnotes are sometimes a good solution and sometimes a bad one; it depends greatly on the tone you want to set. In fact, I'd say the question you want to answer is why your characters should speak another language at all. (The solution with italics would be very confusing—I don't think I've ever seen it done.)
Footnotes will be a momentary distraction to the reader, but will let you specify the word exactly. Terry Pratchett used them extensively, and you can tell he would sometimes make up a word just for the sake of having a footnote that explained it. Guards! Guards! has quite a few examples of this. However, when he uses considerably more made-up words in The Fifth Elephant, he instead uses them with no explanation, or with minor explanation from other characters in the text. They're used there to emphasise that there is a language barrier, and translating everything for the reader would not help with that.
Looking more broadly, in my experience fantasy authors will typically leave the meaning of the terms up to context, or explain them in the narration. If you'd like to see some examples, Brandon Sanderson and Patrick Rothfuss both invent quite a few words. J.R.R. Tolkien is another prominent example, although he does not use them in dialogue all that much and mostly sticks to a fairly small set that are used frequently (despite having considerably more available to him).
Something to keep in mind is that your situation is very different than when people insert terms from some existing language into their book, as Russian authors in the 19th century tended to do with French. These will often be translated in footnotes these days, but it was assumed the reader would know what they are when the books were written. As such, I would be careful of looking at these examples.
EDIT: Oops, I hadn't realised you used a real language—though I wouldn't expect your readers to know Sanskrit, so the situation is pretty much the same. In this case, it sounds like you want your character to appear strange, even by the standards of that world. Adding footnotes to explain things wouldn't improve that effect, so there's no need to. If anything the character says has to be understood (important directions, etc.), you can have a character translate it—after all, supposedly there's someone else who also cares about what it means.
I would use ZERO footnotes in a fictional novel. I think it may have been done, but I think it breaks the reader's reverie and immersion in the story. It is bad form. The same goes for translating in italics, it isn't clear that is a translation, especially if it doesn't happen often.
It all depends on the POV.
If this is the MC and we are privileged to his thoughts, then his thoughts can do the translate for us. But if we (readers) are following somebody else, we should not be privileged to any more information than that POV; and if she doesn't understand "sahu" or that she's been called a pig, then too bad for the reader, they don't get to either. She would understand she's been threatened and likely insulted, and that's enough.
If you are writing in omniscient mode, then the narrator knows what sahu means, and can mention it in prose. I think the literalness of an insult is not important to the reader's comprehension here, almost anything could be substituted for "pig," like "cockroach", "dumbass", etc, without changing the emotional impact at all.
For what it's worth, I also would not put it in all caps; See Manuscript Format for Novel Submission. Publisher's expect emphasis or stress in italics, and specifically do not like ALL CAPS.
I would recommend just leaving it completely untranslated and let the reader pick up the basic meaning from context. In this case, we only really need to know SAHU is an insult and the direct translation is unimportant.
If it is important for the reader to know what a single specific word means, just have another character ask, what you mean by SAHU? Or have the character to say the foreign word and then search for an English equivalent. Those should be used very sparingly though as it would quickly get tiring.
I took riding lessons from a Polish gentleman - former lieutenant in the Polish Cavalry, spent time as a Vaccaro so his credentials were impeccable.
Occasionally, when we were not picking up what he was putting down, he would start to speak Polish. It was our assumption that he was swearing or at the very least, extremely frustrated with us. We would try that much harder to please him. He was an excellent instructor.
Have your character swear in whatever language you choose. The sudden insertion of a word the reader does not know but clearly is of another language - real or imagined - comes across as an insult or swearing.
Many contemporary works of fiction use footnotes. Here are a few:
Look at the books to see how footnotes are employed and to what effect. Here is a long list of fiction that uses footnotes or endnotes. Some of the most famous authors use footnotes and you can find footnotes in some New York Times best selling fiction, so claiming that footnotes must not be used in fiction is both ignorant and foolish. There is even a TVTropes page about footnotes in fiction.
Terry Pratchett often uses footnotes to translate fictional terms. Kate Horsley uses footnotes to translate Latin and Gaelic passages in Confessions of a Pagan Nun. John Green also provides translations in the footnotes in An Abundance of Katherines. The footnotes in The Athenian Murders by Jose Carlos Somoza offer translations and comments by the fictional translator.
I agree with the other answers that translations are unnecessary in your case.
While footnotes are unusual in fiction literature, there are a few examples. The one that uses them for translation that I know is Liu Cixin's "Three Body Problem" trilogy as well as his short stories. They are originally written in chinese. In the english translation, the translator has inserted footnotes whenever a phrase or term or especially name could not be cleanly translated without losing, for example, a double meaning. They are also used to provide context for all of these things.
The thing I noticed is that the footnote is never just a translation. It always is at least a full sentence that gives some context.
Taking from that example, I would suggest the use of footnotes if you need to add details that would break the flow, and are important enough to include, but the reader can without harm decide to jump to the footnote right now, or at the end of the sentence, paragraph or chapter.
