Language use, English-speaking ability, and linguistic isolation data are currently collected in the American Community Survey. In the past, various questions on language were asked in the censuses from 1890 to 1970. The current language use questions, in use since 1980, gather how many people speak a language other than English at home, what languages are spoken, and how well English is spoken.
Native North American Languages Spoken at Home in the United States and Puerto Rico: 2006-2010 (ACS)
Language Use in the United States: 2007 (ACS)
Detailed Language Spoken at Home and Ability to Speak English for the Population 5 Years and Older by States: 2006-2008 (ACS)