Diligent should be an acceptable translation for ahkera.
Diligent should have been accepted
Diligent is absolutely acceptable.
Are the hardworking girl speaking Spanish? Incorrect... why?
Surprised that a question that is simply asking for an answer or explanation gets downvoted. We are learning people! (even if it's an English grammar mistake for a Finnish course)
Hardworking girl = 3ps = she is (not she are)
'Are' is used for " you, we and they". If it was 'girls' it would be 'are the hardworking girls speaking Spanish?'. Because it is only one girl, you must use 'is'. The girl is, she is. You cannot say 'the girl are'. Hope this helps.
I tried "industrious" as a translation for ahkera, but Duo doesn't like it. Hopefully they'll add it.
ETA: And Duo didn't like dilligent, either.
Stuck with the awkward "hardworking" for now, I guess.
Would "Can the hardworking girl speak Spanish?" Work?
Yes but not in this instance
An answer. with puhuko instead of puhuuko should regarded as a typo error.