ICAO Annex 10 CLEARLY states that the language spoken in radiocommunications should be the language spoken in the country. IF in a controlled aerospace an aircraf is unable to speak the local language, English should be used.
As a result, in an uncontrolled environment, Spanish or any other local aircraft in uncontrolled aerospace have the right to speak their own language unless the local CAA legislation tells otherwise.
This has by the way NOTHING to do with the 6 official ICAO languages, that are used to translate official ICAO documents.