"Ils sont pauvres, et ce depuis longtemps."
I wrote "and have been for a long time." which was accepted as correct. I think that's a far more natural translation than the official answer.
I agree, this is a poor translation. No one would ever say "They are poor, and this for a long time" in english. If you want to include this just to illustrate something, at least point this out.
In this sentence, "ce" is referring back to the predicate statement that they are poor (sont pauvres) but there's no straightforward linguistic way to capture the same referential meaning in English. Translating "ce" to "it has been so" accurately references the state of "being poor" while remaining in the present tense.
See the word map of French to English below (the braces correspond sequentially):
[Ils] [sont] [pauvres], [et] [ce] [depuis] [longtemps].
[They] [are] [poor], [and] [it has been so] [for] [a long time].
An alternative translation is, "They are poor, and [that] for a long time" which is clumsy English, but technically correct.
The purpose of the exercise was to point out the fact that the English present perfect with a date or a duration (basically with for or since) translates in the French simple present.
when translating too english I feel like I should change the tense to the present perfect: "They have been poor for a long time." Though this sentence has the same message of the french expression above, I'm pretty sure Duolingo wouldn't take it as a correct answer.
I know exactly what you mean! The translations are full of such clumsy interpretations, things English speakers would never say otherwise. I thought that was the whole point of Duolingo interpretations, to not translate the way computers would. What document was this from? is this translation the 100% answer? A better translation would be: They are poor and have been so for a very long time.