Reading an otherwise enjoyable book this morning, I had the frustrating experience of encountering a chapter which opened with a quotation. Annoyingly, while the book was in English, the quotation was in Spanish and it came without an accompanying translation.
I like to think that, as a former university professor and as an author and editor in my own right, I am a more than averagely literate reader. I can cope with Shakespearean English and the King James Bible with the same facility with which I read Damien Runyan. My linguistic skills embrace Latin and Classical Greek as well as biblical and mishnaic Hebrew and a fair smattering of Aramaic. I can even cope with French menus and street signs. However, the range of languages spoken by human beings on this planet is vast and I have no competence in Spanish at all.
Hillel the Elder had something to say about dropping foreign-language quotations into one's writings. In Avot 2:5 he advises that a person should not say anything that cannot easily be understood if he intends that people should understand it. He, I am sure, would have suggested either translating the Spanish quotation into English in its entirety or featuring a parallel translation for the convenience of readers.
There are many examples of this sort of thing, in print and in speech. One of the most frustrating is to attend a Jewish wedding at which someone stands up and makes a speech -- in English -- the main feature of which turns out to be a long and involved joke. As the climax of the joke approaches, it is clear that it is a real classic and everyone eagerly awaits the punchline. When it comes, it is delivered in Yiddish (another language that I do not understand). Everyone falls about laughing but, when I ask them what the punchline means, they say they can't tell be because it wouldn't be funny in English. I remain unamused.