Translation:You cannot know when they will arrive.
The English translations they offer are all very awkward. The impersonal "one" construction is not frequently used in modern English. A much more natural and commonly used phrase would be:
"It's impossible to know when they will arrive."
It's not a direct translation but the meaning remains the same and it is a phrase that's actually used!
Mainly because the impersonal "one" in modern English is rarely used, is reserved for very special instances, is seen as stylized and overly formal, and is usually replaced by "You" or "We".
You see - I mean, one sees :-), - the same thing in French, where "we can" or "you can" is often translated as "on peut" = "one can". It doesn't really mean "one can" even in French (except literally), but is taken more generally to actual refer to "we" or "you" without using the exact "nous" or "vous". on peut is not a formal way of saying something, it's used frequently in colloquial expressions, just like "you can" or "we can" is used in English.
I'd wager the same is true in Italian - that they do not think in terms of "one can", but use si può as a kind of way of avoiding the informality of addressing someone you don't really know with tu/noi while also avoiding the directness of *voi".
As a result, I don't think that si può is as formal as "one can".
It would be incorrect, because what comes after '... know' is a statement of what is not known and not a question. The confusion comes from the inherent versatility of English words not changing with different grammar constructs. For example, 'when' is a question in the sentence 'When do you arrive?' and not a question in the sentence 'I shall see you, when you arrive.'.
The word-order in English declaratory sentences is subject-verb. [Declaratory sentences make a statement rather than ask a question.]
The word-order in English question sentences is verb-subject - and that includes any part of a compound verb preceding the subject, as in your sentence.
"Will they arrive?" is a question. "They will arrive." is declaratory.
Note that with compound verbs in English questions, the auxiliary verb comes before the subject but the main verb comes after it: "When will they arrive, when do they arrive, when can they arrive, when should they arrive, when could they arrive, when would they arrive, etc."