I disagree. I associate "sincere" with being "heartfelt", and "honest" as being "straightforward". All three words convey an element of honesty, but I tend to think "frank" implies a more emotionally removed dialogue than "sincere". If you have a frank discussion with someone, it is usually short and to the point without additional emotional embellishment. If you express sincerity, I view this as a more passive form of communication
Translation (in straightforward cases like this) is not subjective. The correct translation is whatever the French dictionary defines as the uses of 'franc' (presumably not to mean 'sincere').
In saying that, 'honest' is a synonym for 'frank' and 'sincere' is a synonym for 'honest' – but 'sincere' as a synonym for 'frank' is probably too far removed :)
Another translation for this is "he is straight" which had me thinking that it's perfect timing, considering Mardi Gras is on this weekend in Sydney!
Actually, if someone was blunt, honest, frank etc. I don't think Australians would ever say "he is straight" to convey that. We might say "he is straight to the point", but generally, if someone says "he is straight", the listener is more likely to interpret this to mean "he is not gay".
Maybe people get it wrong a lot? Perhaps because for english speakers in many countries, neither frank nor candid are particularly common words (though not to the point of being obscure) and straight is almost never used in the way duolingo is using it. As someone else pointed out above 'He is straight' means he is not gay. It is basically never used in that sentence to mean the equivalent of 'franc'.
Not a Francophone but I'm guessing in this case "il" means "he" because that's how the expression works: when the adjective describes a person you must use "il est"; when it describes a situation you must use "c'est". Therefore, if it says "Il est frank" it could only mean "He is frank". If you wanted to say "It is frank (though it sounds a bit off to me but I wanted to keep the example)" you'd have to say "C'est frank". Based my assumption on this: http://french.about.com/library/weekly/aa032500.htm See first thing in the chart.
Two late comedians, Johnny Wayne and Frank Shuster, had a skit on their TV show one time, where one of them, was saying something like "I'll be Frank" but it was his name. I don't remember the skit, but one of the lines in the skit was, "I'll be Frank" and it was a joke about using "frank" and his first name was Frank too.
"Frank" is a commonly used word. In a newspaper, the name of a column was "frankly speaking".
A question for Francophones, and probably for linguists, really. I've come across this word in the context of 'francs-bourgeois' - or 'free' bourgeois. Is that an ancient meaning of the word that is now gone out of use? Incidentally, in Ireland we'd use the word 'straight' to mean that someone is honest and won't cheat you, and also someone speaks directly without beating around the bush; the neologism of straight/gay is also understood and used, but is not the primary meaning of 'straight'.
I am not a francophone, but I am a bit of an amateur linguist. The word 'franc,' whence our word 'frank' is derived, comes from Medieval Latin 'francus' via Old French meaning 'free;' thus 'open, sincere.' The original sense of the word was derived from the concept of the Franks, the people group, being freedmen as opposed to the subjects whom they had conquered. Once it had been adopted into Vulgar Latin, thence Old French and subsequently Middle English, its meaning began to undergo normal semantic change to reach its modern meaning. The 'francs-bourgeois' is an example of the older meaning retained in a phrase. Hope this was useful! Have a wonderful day! :)