I´m also a Spanish native speaker and I agree with sticking to English whenever there is non-spanish speaking traffic around. It gives more situational awareness. I spent time abroad and since I started in aviation I did everything in English. Because of that, I have the same issues when using Spanish on the radio. I find it difficult.
Not making a big generalisation, still there are some Captains based on pride insisting on using Spanish because "in Spain, we speak Espaņol". On the first stages of Line training flying I was forced to use Spanish on the radio and being shout at because either I was mumbling, getting stuck, saying too much, or switching to English.
There was even a law about to be passed for using English on major aiports, but the Spanish Pilot Union strongly went against that.