Basque is an official language and declared as such in the Spanish constitution however restricted only to the regions that decide to apply it (Basque Country and Navarra).
If we want to go all legal, I believe that Spanish/Castilian is the only official language of the State, so at country level, with the other "Spanish languages" only official in their respective areas:
Section 3
(1) Castilian is the official Spanish language of the State. All Spaniards have the duty to know it and the right to use it.
(2) The other Spanish languages shall also be official in the respective Autonomous Communities in accordance with their Statutes.
(3) The richness of the different linguistic modalities of Spain is a cultural heritage which shall be specially respected and protected. [1]
