This is the 2543rd most frequent Catalan word.
castella
"Castella" in Catalan means "Spanish," referring to the Spanish language (Castilian).
In this sentence, 'castella' refers to the Castilian language, commonly known as Spanish.
Jo parlo castella molt bé.
I speak Castillian very well.
Here, 'castella' is used in a general sense to denote the language spoken mainly in Castile and widely throughout Spain.
Castella és una de les llengües oficials d’Espanya.
Castilian is one of the official languages of Spain.
In this sentence, 'castella' signifies the language in which the book is written.