True, I totally agree.

However, if one is evaluating "functional literacy" that means determining if one reads well enough to function in society.

So to truly evaluate functional literacy for native Spanish speakers, it seems like one would have to somehow factor in two things.

First, English is the de facto language in the US. Second, Spanish language translations are provided for a number of written things (for example, our school district letters to parents).

One would be more functional being fluent only in English than only in Spanish, sure (and it depends on which part of the country even which part of a city). But one would surely be more function having some knowledge of English and fluency in Spanish.