The United States (or other nation) would need to be structurally-different as to prevent English-language assimilation. I think a combination of a French Louisiana that soon joins/gets conquered by the United States, a more state-oriented government which allows state-level official languages as per each state's constitution (already mentioned in the thread), and the lack of an American expansionalist mentality, like the one that promoted Manifest Destiny and the spread of mainstream American culture to ethnically-different lands (a bit paradoxical considering the previous point), and I think the proposal of more linguistic diversity in the Americas would work.