Languages of Catalonia
The main cause of Spanish and Catalan social bilingualism in modern Catalonia was large-scale migration from the rest of Spain during the 20th century, as Catalonia started a significant industrialization which demanded an increased workforce from elsewhere. Spanish has historically been spoken among a minority of civil servants born in other regions in Spain and among segments of the wealthiest bourgeoisie. Spanish has also been spoken as a second language by most Catalans, as it has been the only official language over long periods since the eighteenth century.
Province of Castellón
It is a bilingual territory whose inhabitants speak both Spanish and the local co-official language of Valencian/Catalan. Although Spanish and Catalan are closely related comprehension is limited: the language is a 'bridge' between Western Iberian and Gallo-Italian. There are marked distinctions in vocabulary and grammar which puts Catalan closer to Occitan in southern France and Italian (in frequently used words) which make it difficult for a monolingual speaker of Spanish to understand. Usage is a source of some controversy and political but not to the degree as in Catalonia.