could they complain about it?
Yes, they can raise their concern with faculty. A glance at a typical instructor's email inbox will confirm that this happens. Certainly, if a student wants their instructor to be aware of something, then telling them about it will be more effective than not telling them.
In this scenario, a student faced with this problem during the exam could also write on their exam paper: "As an American, I am hopelessly confused by the word 'fortnight' and I am assuming it means ten days. On that basis, my calculation is [...]". Or they need not be quite so explicit; if it's clear that they understood the chemistry but plugged in the wrong number for the time period, people doing the grading would ordinarily give due credit.
Another answer suggests asking the invigilator. This is practical if the question is clearly not to do with the subject-matter of the exam (e.g., you wouldn't expect them to explain what titration is), and if institutional policy allows them to give an answer. Universities can have rules about giving mid-exam clarifications, in the interests of fairness and avoiding student stress. (When I was an undergraduate, we had one exam which featured a succession of contradictory clarifications, finishing with the announcement that the entire question would be scrapped. This was very stressful for everyone.) This can also be harder if it is an online exam, and doesn't work if the exam is already over.
In some cases, clearing things up during the exam may well be hopeless. Suppose the subject is physics and the lecturer has set a problem involving the trajectory of a cricket ball, referring to such things as the popping crease, the wicket, being hit for six, and so on. Someone innocent of cricket may find the setup totally bewildering, and the exam hall is not a great place to explain cricket from first principles. The best option here is for the student to proceed as best they can during the exam, and raise the issue afterwards as swiftly as possible through the proper local channels.
Is there anything that can be done to help their grade on the exam?
The processes for this differ between the UK and the US. In America, the professor commonly has very wide discretion to assign grades. British universities will have a different set of methods, with second-marking and oversight by external examiners. But either way, this sort of grading issue is very hard to appeal after the decision is made - there is an academic judgement call, and that is generally that. Student concerns have to be raised within a fairly narrow timeframe, in order to be taken into account.
Students who would like their overall mark to be higher could dedicate their efforts to worrying about a wording issue on a single exam question, and lobbying for a change in grade. Or they could spend the same amount of effort learning more about the synthesis of alkanes, say, and hope to get a higher overall mark in their next module. There is very little institutional appetite in either country for entertaining lengthy disputes about this sort of thing. Matters might be different if the student could show that they were significantly disadvantaged as a result of the grading decision, but this single question will not be the determinative factor that gets them a 2:2 instead of a 2:1.
Overall, even though assessments shouldn't contain hidden cultural assumptions - because then they're not assessing the right thing - the extent to which they can be challenged will vary depending on the seriousness. I have seen universities on both sides of the Atlantic take student feedback seriously, but generally with more focus on improving assessments in the future than on wrangling over ones which have already happened.