The word "castellano" has a couple of different meanings, but in this case I think that it refers to the process by which the native language is replaced by Spanish.
I hope it helps.

I worked with indigenous peoples and when writing about them for a Canadian university, this was the term I was told to use for "castellanización" ... As usual, terminology depends on context and audience ... Hope this helps

Here's a trio of suggestions that take a somewhat different path. I think 'Hispanicize' can certainly work, but it doesn't sound all that natural in English. (My sense is that it most strongly connotes, for most educated US English speakers, the process of transforming a word to make it fit the Spanish language, equivalent to Anglicize. American Heritage Dictionary: "Hispanicize. 1. To make Spanish in form, style, or character. 2. To bring under Hispanic influence or control.")

Also, I don't think we necessarily need to be bound by the part of speech used in the source text, a (Latinate) verb, i.e. formal equivalence. So am proposing to use the more natural-sounding 'to impose Spanish' or, as alternatives, 'Hispanophone' ('a Hispanophone policy' is my own coinage, I think) or 'Spanish-dominant'.

Just two other thoughts: I think CANZUK (Canada, Australia, New Zealand, United Kingdom) is a fascinating acronym. And, I'm reminded by the story about Spanish Minister Wert of Borges's famous crack about Spaniards loving the Spanish language "because for most of them it's a foreign language."

"José Ignacio Wert, as the Financial Times reports, announced plans to ..... a nationalist (Spanish nationalist) by wanting to impose Spanish on Catalan schools...?" www.economist.com/blogs/johnson/2012/12/language-nationalis...‎

Mabel Moraña. Ideologies of Hispanism. "... before: impose the Spanish language and customs while battling the other s languages ... Children were drawn into church schools not only to evangelize and ..." books.google.com/books?isbn=0826514723

"As regards Canzuk... having a more of a melting pot attitude to culture (to the point that it could become Hispanophone in future), and having a population twice the ..."
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