"Whose is the turtle?"
I would say the correct English translation should be 'whose turtle is it?', no?
Apart from that, I'm guess the literal translation of this 'kaplumbağa kimin?' is 'turtle whose?', or more... 'turtle whose is?', right?
And constructing 'kimin' as a genitive... 'kim'in'... like... Baris'in... only you take out the apostrophe because 'kim' is not a name.
Or maybe I got all wrong.
I asked my mum because turkish is her first language , and she explained it to me like this:
Because Turkish uses a Subject-Object-Verb sentence sture, saying kaplumbağa (turtle) FIRST establishes it as the subject. So it becomes obvious that you are talking about the turtle in front of you.
Whereas if you don't say it first, then you must use the specific term (kaplumbağası) to distinguish that you are talking about THIS turtle rather than the general concept of turtles.
Sorry if this isn't explained very clearly.