"Spanish conversation always starts with the pilot calling in Spanish. It is not the controller who selects the language".
This is complete rubbish I'm afraid. My last company used to fly subservices for a Spanish airline and we were required to use their callsign.
We always checked in using English but a hell of a lot of controllers always came back with a torrent of Spanish as soon as they heard the callsign.
It took me three attempts one day to get the lady on Ground in Madrid to talk to me in English. I finally got through to her by stating that I was a gringo and would she please use English.
That did the trick!