Was the official language of Spain always Spanish?

Firstly, 'Spanish' does not exist. We call the dominate language in Spain, which many refer as 'Castillian', Spanish.

Castilian was just one of many Romantic languages which existed in the Hispanic peninsula. This language became the dominate language in present-day Spain due to marriages (Ferdinand and Isabella) and due to the fact the Reconquista (driving the Moors out of Spain) was led by the House of Castille.

In present day Spain there are still a few other languages spoken besides Castillian: Catalan, Galician, Asturian/Leon and finally the world's largest language isolate, Basque (which is not an Indo-European language).

During the very oppressive Franco dictatorship (1939 to 1975), Castilian was the official and universal language and any 'official' usage of any regional language was banned. All media and education had to use only Castilian. Since 1975, regional languages have thrived in Spain.

For yet a clearer picture about the history of Spanish (or Castilian), I would recommend the book 'The History of Spanish' written by the bilingual Canadian couple, Nadeau/Barlow.