"uses data from the 2000 United States census to display the locations and numbers of speakers of thirty languages and three groups of less commonly spoken languages in the United States. The census data are based on responses to the question, "Does this person speak a language other than English at home?" The Language Map illustrates the concentration of language speakers in zip codes and counties. The Data Center provides actual numbers and percentages of speakers and includes census data about seven additional groups of languages less commonly spoken in the United States."
Cool reference tool!