A stronger or otherwise different conflict between Protestants and Catholics in the U.S. might help. Know-Nothings or their equivalents succeed in getting some nativist laws passed, including establishing English as an official language (or at least on the course to becoming one). Catholics with political clout respond to these actions. One response involves German Catholics in the Midwest making German a secondary official language in their local communities, if not one or two states with German majorities. The inevitable disputes happen to reach the Supreme Court during a period when liberal judges are in power, and the ability for those communities to maintain this status is upheld.
Of course, as the immigrants incorporate themselves more and more into U.S. culture, and fewer and fewer people speak German, the requirements that public documents be written in both languages becomes more and more of a liability, and while German remains a de jure official language of Milwaukee, de facto the civil servants tend to ignore it.