Thank you for the history lesson...
After Texas became part of the US, people in Texas still learned Spanish to make communication easier.
There is no evidence that not having a national language is hurting the US in any way. So why change?
We have a large and growing group of immigrants, mainly hispanic, mostly from Mexico; many of whom are here illegally; many of whom are not intrested in assimilating and "Becoming American" but simply reaping the benefits of our system; many of whom are making little or no effort to learn English.
How do you get by in a country when you don't speak the most common lanugage? Answer: you hang out with those who speak YOUR language; do biz with them, work for them, identify with them. It is yet another way of creating schizm within the nation, where you have a large and growing group who does not identify with the mainstream culture at all, doesn't speak the language, and doesn't really care as long as we bend over backwards to accomodate THEM.
Too much of this and we really will become "Two Americas"... or three, or four, etc.
Maintaining a sense of "we're all Americans" can be difficult enough in the diversity that this nation has.... lacking the ability to SPEAK to each other is a good way to finish off any sense of fellow-citizenship that might have grown between this and that group.
When you walk into the middle of a group of people who are speaking a language you don't know, what is your gut reaction? Do you think of them as fellow Americans, or assume they are "other", "alien", "foreigners"? Most people assume the latter. There's a reason for that.
How do you deal with people with whom you cannot communicate? They don't speak your language and you don't speak theirs? The truth is, most of the time you shrug and walk on. Since you can't communicate with them, your ability to interact with them is severely limited.
That's no way for the citizenry of a nation to live, unable to communicate. That's why we need a common language. The proliferation of immigrants both legal and not who are NOT assimiliating is why we need to make it official.