Inspired by this question.
In many translated works (the first to come to mind are translations by Richard Pevear and the English versions of Isaac Bashevis Singer's writings) there are elements of the original language which is kept, while the bulk of the text is rendered into translation. Why is this done and what purpose does it serve?
The purpose of keeping parts of the source langue no-translated, or using some variant or dialect of the target language for some elements (eg: dialog, a specific character talk, ...), depends on the work, its creators (author, translator), and the languages involved. But, here is some general reasons and uses that I can think of:
The original text, is based on a cultural reference that doesn't have an equivalent in the destination language
the text represent a title. or an honorific (cf. Japanese honorifics)
it's a location name.
it is a maxim a motto, or a proverb.
e.g: "Carpe Diem", "Unus pro omnibus, omnes pro uno", or:
“...L’amour fait les égalités, et ne les cherche pas."
-- The Red and the Black, Stendhal (Henri Beyle), translated by Charles Tergie
it hold a mythological, spiritual, or religious meaning
e.g: "chants" and "prayer", are rarely translated.
the text is constituted of poem verses, it is a poetic prose, or it sound way more elegant in its source language.
e.g:
‘Vous qui pleurez un passé plein de charmes, Et qui traînez des jours infortunés, Tous vos malheurs se verront terminés, Quand à Dieu seul vous offrirez vos larmes, Vous qui pleurez!’
the original text is in a variant of the source language other than the standard one,
e.g: countryside language or a spoken unofficial language.
the author or translator opted for this trope:
Bilingual Bonus: A hidden message in a foreign language. This ostensibly makes these messages available only to bilingual and international audiences. The extra can be anything from a plot-relevant point to additional dialogue to a random gag.
-- tvtropes
@yaitloutou has a great answer, but there's three reasons they've miss out on that I'd like to include here:
To convey a change in language in the original text: possibly the most famous example of this is Shakespeare's Et tu, Brute? from Julius Caesar. Shakespeare's source for the historical incident, the Roman historian Suetonius, has Caesar switch from Latin to Greek to say "καὶ σὺ, τέκνον;" ("You too, child?"). In translating this scene to English, Shakespeare kept the language change, but translated the Greek into Latin so that the line would be understandable to his audience, but convey the sense of a man anguished in his last moments of life (or, in an alternate interpretation, "you too [will die], Brutus").
Atmosphere: Including language specific to a part of the world can make the fictional world feel richer. Your example of Isaac Bashevis Singer is a great one here, as he has been described as using Yiddish to conjure up the atmosphere in the shtetlekh of Central and Eastern Europe, as in Tablet in 2012:
Most remarkable of all, however, was Singer’s ability to go on to producing fiction in a language of ghosts—stories dealing with a dead or dying world that were nonetheless living works of art. No wonder that as a Yiddish writer after 1945—the only one known to most American Jewish readers—Singer was regarded as, and called upon to be, a representative of the Old World, a medium channeling a perished Yiddish culture. After all, wasn’t his work itself filled with mediums, ghosts, and spirits, with dybbuks and demonic possession—all the paraphernalia of a vanished superstition? Where but in Singer’s pages was this lore kept alive?
In translating Singer, translaters may keep some Yiddish words to keep a flavour of both Singer's original Yiddish writing and so indirectly to convey the culture Singer is writing about.
Macaronic languages are fun, and often the only way to translate them is into other macaronic languages.
