The U.S. Census Bureau just published a new report on languages besides English used among Americans.
The report is an excellent read, but this map is the real standout.
You'll see the regions where non-English speakers are among the highest numbers of the population. When you compare it to the map of the most populous counties in the U.S., it becomes abundantly clear that while English may be the language of commerce and government in the country, it'll never be the only language.