This is exactly wrong. The "onus" is on the examiner to make the question clear. When they don't, in such a case, it is just a case of cultural arrogance. "My language is better than your language." Do I need to become a native British English speaker in order to begin any class in UK? And do I need to understand every nuance and variation?

@Buffy: So we reduce all teaching to "simple English"? No, this answer is spot on: The "default" language of a course is the default teaching language of the university, which is usually the local language unless otherwise specified. Obviously, students should be given the opportunity to ask clarifying questions, which is useful for non-language-related issues as well.

@Heinzi, I welcome you to come to Mississippi or Alabama.

Where I "stop" is where I begin. The onus is on the instructor. In fact, I'll just guess that "fortnight" is somewhat imprecise, not exactly 336 hours as would be required for a technical question about a chemical reaction. "See you in a fortnight" has a lot of slop in the intention, I'll guess. Any decent instructor should recognize if/when they have non-native speakers and be sensitive to that in exams.

@Buffy: Well, my wife had to learn some Catalan for her Erasmus stay in Barcelona. Stuff like that happens, and we can't expect the world to reduce their vocabulary to the international language of "bad English".

@Heinzi, yes "some Catalan", but not the nuances of slang and all its variations.

Heinzi is correct: fortnight is not slang in British English. It's no more unusual a term than "week", "day", or "month". And no less precise than saying "14 days later".

@JackAidley It’s orders of magnitude rarer than those terms.

I looked into some list for C2 vocabulary (the highest level of language proficiency) and fortnight is not on them. It’s rarer than some words on the C2 lists. I am not surpised since you rarely need to specify a period of two weeks for anything. My native language (German) does fine without having a specific word for this at all.

Maybe the non-British English speakers should not try to argue with the native British English speakers about whether a word is commonly used in British English... I'm with @JackAidley here: fortnight is on the same level of comprehension as "week". It is in no sense an obscure word. @Wrzlprmft "you rarely need to specify a period of two weeks": maybe you rarely specify this in German precisely because you don't have a word for it.

@astronat Mind that I am not arguing based on my personal experience but on evidence. Week is used about thirty times as frequently as fortnight in British English writing.

@Wrzlprmft: I'd consider fortnight in English about equivalent to "übermorgen" in German. Just because English doesn't have a word for "the day after tomorrow" doesn't make in unusual in German

Yes, fortnight is less commonly used than day or week but it's still within the category of extremely common English words, and @astronat has a very good point about the relevance of non-native speakers here.

Hell, here in Nordfriesland, my recycling bin is collected fortnightly. It's not like this isn't a concept that comes up frequently in Germany

@JackAidley I am not saying that it doesn’t come up ever, but given the fact that our lives are mostly organised in terms of days, weeks, and months, it’s considerably rarer. If a word cannot be translated to another language that is a good indicator that the underlying thing either has culture-specific relevance or is not very important.

I think it's fair to say that someone who is (1) a native British English speaker, (2) an academic, and (3) hasn't particularly directed their mind to the question, would be likely to overestimate how well students would understand words like "fortnight".

Also, we don’t even need to turn to German, because there is American English. If fortnights were somewhat important, why did American English lose this word?

but I think this is all a bit of a distraction from the substance of the question, which is a hypothetical not limited to the specific word. I'm sure we can all think of weird cultural idioms until the cows come home.

@alexg Well, it matters to some extent as it helps to have some estimate of the level of frequency of the word in question. Fortnight is certainly more frequent than widdershins, but then it is also less frequent than week. We would probably all agree that a student can be expected to know week, but not expected to know widdershins. Fortnight becomes interesting on account of being somewhere in the middle.

@Wrzlprmft: I think your answer dodged the point. Would you think that a German instructor should avoid words like übermorgen or doch?

@JackAidley It’s a bit hypothetical since: 1) I can’t see an opportunity for using that word in an exam (just like morgen or gestern), 2) English is the language of instruction here after the bachelor’s level, if not earlier. — That being said, I would probably not consciously avoid it, but also not hesitate to answer it if it comes up.

I would also not mind anybody bringing a dictionary to an exam (if I can somehow ascertain that it’s not full of cheat sheets or similar).

For all I have learnt about the usage of the word so far, I would guess that fortnight is a typical social marker distinguishing native speakers from those who learnt the language later on. I knew the word before this Q&A but I am quite certain nobody bothered to teach it to me in school and it’s not even on the respective vocabulary lists for high-level language exams. I consume a lot of British media and I hardly ever encounter it there.

By the way, even British literature appears to prefer two weeks over fortnight nowadays.