Bernstein: "The immigrants of 1880-1900 radically subverted the language environment of the northeast and midwest as non-English speakers began to settle here at an almost geometrically escalating rate. By 1900, according to Peter Quartermain's assessment, about one quarter of the white US population either did not speak English or learned it as a second language--while in the mid-Atlantic states and New England perhaps only one person in four was a native speaker of English"
Quartermain's introduction to Dictionary of Literary Biography vol 45
he derives this from Census Bureau reports and Reports of the Immigration Commission (1907-1910)