Who 22:55 May 31
Maciej, we use "siapa" or "siapakah" to ask about a person, mostly about their name or what have they done. The suffix "-kah" at the end of the sentence didn't give any significant difference in the translation meaning. The 2 answers provided by Henky and Pipit down below are correct.
Who did this? = Siapa yang melakukan perbuatan ini? / Siapa pelakunya?
Good luck :D | 
peer agreement (net): +3