In principle I think it's un-democratic to force english only in all government & education. If through immigration or whatever, the US were to become 51% spanish speaking, and the spanish speaking majority prefered that ballots, etc. be bilingual, then I see that as being entirely consistent with being a true democracy. In my mind, the same holds true at a district level, especially for voting on local issues. The 'culture' that needs to be protected in the US is the tradition of acceptance, being a 'melting pot' country that has always had a flow of non-enlgish speaking immigrants, many of whom live happy lives here without ever learning a word of english. Saying, 'it always has to stay english by law so we can prevent a time when it won't be english' is an ethnocentric, cowardly approach, and I consider those who strongly feel that way to be not too far seperated from the KKK and other white supremacists in their motivations. (Jeez, that's asking to get flamed isn't it - I'm thinking more of the people who have actually mailed me flyers about 'english only' movements, not those participating in this thread so far)
In practice, it's clear that anyone living in the US present day (as opposed to a hypothetical future) will have a much easier time carrying out their civic duties (voting, jury duty, visiting montana, etc.) if they have a good grasp of english. Immigrants in the past have made sure their children learned english because that was the practical thing to do. It still is the practical thing to do. You might be able to lead a happy American life from cradle to grave without a word of english if you are born in a neighborhood where a US minority is the neighborhood majority, and you're content to live your whole life in that neighborhood. If you want to participate as a US citizen outside of the context of that small neighborhood, you still need english.
Btw, I'm a native english speaker, bilingual in french due to a french immersion program in school.