In Spain if you are making a denuncia in relation to a criminal matter it is to the National Police or the Guardia Civil. Non-criminal then the Policía Local. After they take any immediate action, which is necessary, and satisfy themselves there is a matter to be answered, the papers are submitted to the Court. A judge then decides what action or further action will be taken.
A denuncia can also be made directly to the court. The advantage in the case of non-Spanish speakers is that court will have a qualified interpreter available.
This is also often the best way to proceed in complex or complicated cases.
For more info see: -
http://www.decottalaw.com/articles/Spanish-Denuncias-Questions-Answers.pdf