Castilian ia a region of Spain where the "Spanish" language comes from. But there is not the only language there, and there was repression against the other languages there, specially the basque language.

So, call that language Castilian ia also recognize that other languages exists within the country.

(I'm not spanish nor a speaker, so, I may be wrong)

I like this one:

Dekatrian Calendar (translated to english)

Calendário Dekatrian (original)