The 24-year-old Brazilian-born striker has spent his entire senior career in Europe.
Costa has revealed that they are just awaiting confirmation from FIFA about his switch of allegiance.
"I don't get involved. What's most important for me is to keep on working," he told AS.
"It's up to FIFA to make the decision. The decision to play for Spain has already been made.
"Spain has given me everything and if I can play for the national team I will do it as well as I can."
Brazil's FA vice-president Marco Polo del Nero told Estado de Sao Paulo: "The Brazilian FA believes in free will, if a player wants to play here or there, it will not be this organisation that will say he can't."