The Constitution specifically mentions providing for the common defense and promoting the general welfare. It also gives the federal government the power to regulate all matters concerning interstate commerce. That being said, here's a bunch of reasons to enforce English as the official language of the United States of America and require that any and all immigrants learn at least the basics before entry.
1. A multilingual society harms the economy. Look at the intruction manuals in internationally distributed products. They cost 5+ as many times to make, since they are printing in at least 5 languages (English, French, Chinese, Japanese, and Spanish). Now think about how much money we waste every year making sure those criminal dolts among us can understand what's printed and said. There's interpereters, extra paper, extra ink, and then the ridiculous amount of time confused Americans spend trying to puzzle through transactions with invading Mexicans.
2. A multicultural society leaves us open to attack from within. One of the things any new citizen must do is swear off any allegiences to the old country. How can we begin to think they're for real if they won't even learn our language. You can't show loyalty to someone with whom you cannot speak. You see all this stuff about Aztlan? If it came to blows, which it may, who do you think the Spanish only crowd will side with, the side of the Anglos they can't communicate with or the side of 'their own' people.
3. America cannot stand divided. Even if all of these communities from other countries were loyal to us, America cannot live with itself when it's not united with itself. My hometown has a large chunk of it owned solely by Mexicans. As far as I'm concerned, it's not even there. Nobody there speaks English. Nothing is printed in English. It's also more ridden with crime than any other area of the town, mostly against the white people who foolheartedly or mistakenly wandered in there and made the mistake of getting out of their cars.
This country must be one, united whole, not a series of disparate communities with nothing in common other than the political and geographical borders they live between.