All that said, though, my biggest problem is this: how the heck do you define "English" enforceably? Good law should be crystal-clear and well-defined, so you can unambiguously say "This is legal, but that is not." But English is anything but well-defined: it's the loveable mutt of world languages. I mean, the winning word of the National Spelling Bee was "ursprache", for heaven's sake. From grammar to vocabulary, English is the slipperiest of languages, melding and mixing with everything it comes in contact with.
Okay, yes -- there are sentences that are clearly English, and others that are clearly Spanish. But Spanglish is a very real phenomenon, so this isn't an academic question. Is that English? How much Spanish is too much to consider it no longer English? Where are the boundaries of the English language?
The classic example of a language that enshrined itself is French. Once upon a time, it was the world's language -- there's a reason for the term lingua franca. And it got so full of itself that it went and declared that it was "official". That inevitably led to a large, messy and sclerotic bureaucracy to regulate the language, defining what was "French" and what wasn't. And guess what? It wound up losing out to the vastly more fluid competitor, English. Yes, national power has a lot to do with that, but so does English's adaptability, and the way it is constantly (and rapidly) evolving to keep up with the times, and with the cultures it encounters.
Hence, I find myself opposed to the idea of formally making English the official language of the country -- not so much because the idea is fundamentally daft, but because it invites a problem of definition that we're better off leaving alone...