Denuncia translated basically means “Complaint”. Making a denuncia is an important part of the Law and Order system in Spain, and is a lot more serious than making a complaint as in the english word definition.
Once you denounce someone you should consider that the matter will likely end in court. Whether you are the person making the complaint or have been denounced, you should seek legal advice.
Most "denuncias" are made to the Guardia Civil which is the oldest law enforcement agency in Spain. It is organised as a military force charged with duties under the authority of both the Ministry of the Interior and the Minisrty of Defence. Most Guardia Civil or Police stations do not have interpreters, so take one if you do not speak good spanish. If it is a criminal complaint, it is advisable to use a lawyer both for the language and your own protection.
The State will provide a lawyer for some cases. In Spain, those who make false statements can be punished with fines or even prison, it is considered a crime under “Article 456” of the Spanish “Criminal Law”.
Being another nationality does not put you at any disadvantage in Spain, you will not be treated in a different way either by the police or at the court because of your nationality, but do take your passport as your main means of identification together with Spanish residency (NIE) if you have. Good luck!!