Hola!
Keep in mind that we ATCO´s use the language that the pilots start RT with. We can not make a pilot use Spanish or English if he doesn´t want to, but we have to use one or the other if the pilot makes that choice.
Many times I would prefer to use only English, specially in very complicated and busy moments, instead of having to remember which language each pilot chose to use.
In short: it´s not always us ATCO´s who to blame if English is not used in all RT.
Saludos!