I bet you Alvarez used Spanish which wouldn't be obvious to English-speakers on purpose. It's a defamiliarizing tactic, a linguistic reminder that you're reading a book written from a particular perspective. Another function may be to mimic the experience of being in a foreign culture, not knowing what's going on, literally not understanding. And lastly, I don't know how much experience you've had with foreign languages, but translations are always inaccurate (probably the things which translate worst are poetry and colloquialisms). Using certain Spanish words and phrases may make her writing more accurate, more truthful.

It doesn't seem like a very good way of transmitting unfamiliar ideas and concepts to people who don't know the language or the culture. I always thought that clarity and making concepts comprehensible to the reader were part of a writer's job. What's the point of making sure that part of your audience DOESN'T understand you?

(This is not a problem solely with Alvarez. I've had similar difficulties with other authors--Agatha Christie comes to mind, as she's inclined to let Hercule Poirot explain matters to Hastings in French for whole paragraphs. Which is fine, if you speak the language. It's not so fine if you don't.)