American culture has always been shifting as you say, but while there have been important minority languages in the past, like German especially, it's a different situation now that communication technology and globalisation have so changed the US - and the world. And it's different in that there's a single language now that is threatening the overall dominance of English. In most places it's not a real problem as the Spanish-language speakers need English to get by, and will learn it well enough to speak it to people from different ethnic backgrounds. But for those people who live in neighbourhoods where the Spanish-language majority becomes so great and so dominant that English largely disappears from public life - as I've heard said is the case in parts of Miami and doubtlessly other places - it's quite understandable that they'd feel threatened. Just like those Flemish people in the Brussels suburbs who resent the large numbers of French-speaking people who neither need nor want to learn the local language, and resent the feeling of becoming a cultural minority in their own region. The other aspects of culture aren't much of an issue in either case, I think - in the US because American culture, especially in the border states, already has so much Hispanic and other foreign influences anyway, and in Belgium because in the end Flemings and Walloons aren't that different in most regards.

Highly educated and wealthy people - particularly if they are multilingual, or otherwise inclined to be cosmopolitan - are always going to be more comfortable with such a situation, because it doesn't feel as threatening to them. But that doesn't mean the problem isn't real or shouldn't be taken seriously.

Eh. I think it's fairly comparable.