The United States does not have a national language. The need for one includes:
1) Requiring minimal proficiency for citizenship
2) Requiring minimal proficiency for diploma/degree
3) Eliminate waste on multilingual forms for things only citizens can do (ballots, application for US passport, etc.)
Note: This should not discourage diversity. Schools should be encouraged to offer sufficient education for high school graduates to be fluent in at least one alternate language (Spanish, Mandarin, etc.) as the district chooses.