This is not a very satisfying answer, but it is something that his highly dependent on field and even subfield.
In the physical sciences, for instance, roughly from the post-War period new discoveries were generally published in English. (I believe many inter-War break throughs were published in German, as an example.) /u/restricteddata has an excellent answer in the thread:
More fun little notes in this thread:
In sociology, knowledge of German wasn't exactly expected but you'd regularly see citations to untranslated European originals I believe into the 1940's at least. Weber's famous concept of rationalization as an "iron cage" dates to a 1930 translation by Talcott Parsons (another famous sociologist in his own right) of Weber's 1905 he Protestant Ethic and the Spirit of Capitalism, and I believe that was the first translation of the seminal work. Robert Merton, as another example, one of the most famous theorist of post-war "theorists" translated some Durkheim from French. C. Wright Mills (a famous sociologist in his own right) translated some previously unavailable works from Weber in the 1940's, and I think the rest of Weber's works were mostly translated by the 50s (keep in mind, Weber died back in 1920). While not all sociologists of that era were expected to know French and German, this seemed to have been somewhat expected of "theorists". This was am mostly coastal tradition, however, and I can't recall seeing many foreign language citations in the early, more indigenous "Chicago School" of sociology.
By the time we get to the Critical Theorists and Post-Modernists in the 1960's, continental European books were almost immediately translated into English or they were unimportant. Even Foucault's first major book, Madness and Civilization, was translated into English within like four years (the translation was of an abridged second edition published in France the year before). I don't have firm dates on the matter, but I believe that some sociology graduate departments still had foreign language requirements into the 1960's. I do not think any program does now. The last major sociological "grandfather" to have all their major and almost all their minor works come out may have been Marx--the Marx-Engels Reader, now ubiquitous in introductory college courses, came out in 1978 I believe. This isn't to say important work necessarily comes out in English original after that. Nikolas Luhmann (who died in 1998) is perhaps the last sociologist who is considered particularly important and wrote primarily in German I can think of, the generation of Pierre Bourdieu (died 2002), Jean-François Lyotard (died 1998), and Jacques Derrida (died 2004) might be similar in French. Those last four names, however, were primarily available to American sociology in translation. We moved from a period where some foreign language knowledge was considered important before the War, a period where everything was available in translation, to a period where almost everything that strives to claim global importance is published as an English original.
Some fields are much more conservative. When I did my undergraduate in religious studies 2003-2007, many of the texts we read still had bits of untranslated French. This is facilitated by the fact that even American PhD programs still frequently require both French and German regardless of what you study. Some, like Stanford's, will let you substitute "relevant language of secondary scholarship" (if you study Japanese Buddhism, you can learn Korean and Chinese), but some, like the University of Chicago's, still demand French and German from everyone. Important new arguments in Biblical studies are still published in French and German (and, for the Hebrew Bible, in Modern Hebrew), and in some fields its still very common have a large portion of the bibliography be French or German. One scholar I like, Bruce Lincoln, is an American who works on religious aspects of the Achaemenian (among other topics) has published books in French as recently as three years ago because that's one of the major language of Achaemenian scholarship. In some of his research papers (and other religious scholars), it's certainly not rare to see quotations from other researchers go untranslated as in, "As Fourier (1994) argues, 'Ce texte ici n'est qu'un exemple de ce dont Je veux expliquer et je suis désolé si vous avez pris le temps de le traduire.'" I had a friend in college who wanted to learn one specific ancient Mesoptamian language (Akkadian, maybe? something like that--not one of the big ones like Sumerian, Babylonian, or Assyrian) and claimed she needed to learn German first because the only textbook available was in German. This sort of language requirement is still strong in religious studies, but if I had to guess, I would say it was more commonly used in the past than today.
Within popular culture, this question is harder. In novels, some of the time chunks of foreign language in French or more rarely German is meant to convey the pompousness of the narrator. Then, you have books like Gravity's Rainbow which I've never read but I know includes several untranslated chunks of German because my roommate would ask me how to translate them. The reader doesn't seem necessarily expected to understand.
You may also be interested in my older answer for:
It seems that this expectation of educated audience was certainly expected in the 19th century, but it's very hard when to say it was no longer expected of an educated general audience. I've certainly encountered it in fiction (I remember it in one Daphne Du Maurier book from the 1960's), but here we get into a question of authorial intent. Does she need us to understand the sentence, or is this window dressing which shows what kind of person her character is or what kind of place they are in? I'm reminded of the contemporary fiction by authors like Junot Diaz which have Spanish (a language I don't speak) mixed in them, and subsequently have to ask my Spanish-speaking friends about. Diaz doesn't necessarily expect an "educated person" to speak Spanish, but he uses it to add characterization to his stories. Judgements of this kind, that rely on authorial intent, are quite hard.