Of COURSE foreign language should be required, regardless of whether or not we have large populations of immigrants who speak any other language. English has its roots in lots of other languages. I don't know that I can think of any aspect of english, in fact, that is NOT rooted in anything else. People complain that so many people don't know enough of english to learn anything else, I say that learning something else will help with english comprehension, too. Often, learning things in tandem helps make them easier to understand. I think we don't start teaching foreign language early enough. The earlier you learn, the easier it is. Did you know, that all humans are born with the ability to produce the same sets of sounds? However, as we hear certain sounds and combinations, those are the ones we practice, and sort of lose the flexibilty necessary to do them. If we had practiced as children, for instance, the rolling rrr's in the back of the throat as in spanish or french would be a great deal easier for us as adults. Full comprehension isn't necessary at an early age, but familiarity is so helpful.
It also disappoints me that Latin is so rarely taught as a foreign language. Latin, being the root of most of the European languages, not to mention the language of medicine, is so very helpful to know.
No only that, but while english is the common language of aviation, french is still the common internationally used language of politics. Business used to use english as well, but the global economy is changing, and chinese is becoming more and more important.
As for the other issue, of learning spanish to accomodate immigrants, no - I don't think it's a necessity. But it's a smart business move. People are more comfortable speaking what they speak at home, whether they are proficient in another language or not. If you can speak to them in that language, they will do business with you. If you live near a large Korean community, you pick up a little korean. If you live in certain parts of Alaska, you might pick up some Inupiaq. This is how english got to be a conglomeration of other languages in the first place, and no doubt it'll continue. True, some more spanish words will inch into united-states english, just because spanish-speakers are a large immigrant population right now, but that's really no different from the rest of American history.
I agree with some of the others' observations that speaking a language and reading it quickly are not the same thing. You yourself, misspelled "Frere Jacques." (I'm not meaning to nit-pik, mind you, but it re-inforces the point - and yes, I am aware that I technically misspelled, it, too, as I left off the diacritical mark... :) )
Side-note for the road signs, even the drivers' test in english requires the student to identify them, not by what is written on them, but by color and shape. They are color- and shape-coded on purpose to make reading them unnecessary.