I know it's not a literal translation but in british english we would probably say
Could you turn the computer off.
And to be totally pedantic and knowing that ..
Can you turn off the computer is nowadays sometimes used....
Can you turn off the computer is a question.
Could you is also wrong but more likely to be used as could you is also a question
The most correct form would be
Would you turn of the computer
I know that you will probably say that would is the past tense of will
But would and could are not so simple in use. They are also used as a polite way of requesting something not as overly polite as for instance
Just everyday normal polite like możesz