* Spanish is the official language of Spain: Virtually everyone in Spain is exposed to and learns Spanish.
* Regional Languages: While Spanish is the dominant language, Spain also has several recognized regional languages (Catalan, Galician, Basque, etc.) Many people in these regions are bilingual in Spanish and their regional language.
* Immigrants: Spain has a growing immigrant population, and not everyone speaks Spanish as their native language.
Therefore, it's safe to say that a vast majority of the population in Spain speaks Spanish (likely over 99%).
However, it's misleading to say that everyone in Spain speaks Spanish fluently, as there will always be some exceptions due to age, immigration, or individual circumstances.