"We cannot say that word?"
Translation:To slovo říkat nesmíme?
Interesting observation. As a native (US) English speaker, I wouldn't expect to hear anyone say, "Cannot we say that word?" US speakers would almost certainly use, "Can't we say that word? or "We can't say that word?" Maybe your version would be more common among "British English" speakers?
Well, for me "We can't say that word" sounds like affirmation, definitely not as a question. Although I'm not a native speaker, but we were taught the question sentence should be started with either question word or with verb at the first place. So I would expect the question: "Can't we say that word?"
"We can't/cannot say that word?" is what is known as a declarative question, which are also used in English: "A declarative question is a yes-no question that has the form of a declarative sentence but is spoken with rising intonation at the end. Declarative sentences are commonly used in informal speech to express surprise or ask for verification. The most likely response to a declarative question is agreement or confirmation." Link: https://www.thoughtco.com/what-is-a-declarative-question-1690372
Yes, but to revisit my point again, is your contention we could use "we must not" in this exercise not really still an interpretation of "We are not allowed to"? I'm labouring this point because of the need for avoidance, as far as possible, of ambiguity in translation exercises. I need to get a sense of the true meaning of the Czech!
The original sentence is the Czech one. When doing the reverse exercise, that is translating the Czech sentence to English, you can use: [cannot/may not/must not/are not allowed to/are not permitted to] and all of them are accepted as correct. I suppose the reason somebody chose "cannot" as the main translation is the one I mentioned previously - it's the most common way to express the same idea in English.
But I agree we should have more sentences, more examples from which it would be clear that "nesmíme" means "we are not allowed/permitted to", not "we are unable because we don't have the skill or ability or knowledge" which "cannot" can also mean.