Names given to the Spanish language
Formally speaking, the national language of Spain, the official Spanish language, is the Castilian language (as opposed to regional Spanish languages like Galician, Catalan, Asturleonese and Basque). As such both names, "español" and "castellano", have distinct and independent meanings that may be required for clarity in some specific contexts.
Generally speaking, though, both terms can be used to refer to the (national) Spanish language as a whole, with a preference for one over the other that depends on the context or the speaker's origin. "Castellano" (as well as "Castilian" in English) has another, more restricted, meaning, relating either to the old Romance language spoken in the Kingdom of Castile in the Middle Ages, predecessor of the modern Spanish language, or to the variety of Spanish nowadays spoken in the historical region of Castile, in central Spain.
Erromintxela language
There are currently an estimated 500 speakers in the Southern Basque Country in Spain, approximately 2% of a population of 21,000 Romanis, and another estimated 500 in France. In Spain the remaining fluent speakers are elderly people mostly over the age of 80; some are equally fluent in Spanish, Basque, or Caló. Middle-aged Erromintxela are mostly passive bilinguals, and the youngest speak only Basque or Spanish. In the Northern Basque Country, however, the language is still being passed on to children. The percentage of speakers among Spanish Erromintxela are higher than 2%, as large numbers of Caló-speaking Romanis moved to the Basque Country in the intense period of industrialisation in the 20th century.