There are plenty of places where it could happen, given the right POD. Traditionally French-speaking areas like Louisiana and parts of northern New England could retain their French. A larger and more insular German-speaking population in Pennsylvania and New York is another possibility. The Spanish-speaking southwest is another fairly easy one.
Now if you wanted to get REAL creative, you could wank an Indian nation--Cherokees, Seminoles, Navajos, and Comanches all sound like interesting choices off the top of my head--to make them somehow retain their language in much greater numbers than they do today.
Or you could do some sort of Southern slave-wank and create a large population of free blacks speaking a creolized dialect of English, sort of like Gullah but with way bigger numbers.
But ultimately the key to a USA that recognizes languages other than English--whether it's de jure recognition or just de facto--is numbers. You need enough people that use this as their primary language that they can't be ignored. For example, businesses in New Mexico pretty much have to employ at least some people who can speak Spanish; it's an economic necessity. Cajun French in Louisiana? Not so much.