The Federal Government has of course had no language policy at all regarding non-indians, and in the 19th century, much of the US was effectively bi/tri-lingual at any given time.
Nativism and WWI of course changed that, with 15 states banning Non-English instruction during or after the war till it was struck down in
Meyer v. Nebraska. It still killed bi-lingual education in the states.
Kill the English-Fluency=American Patriotism meme early on (in about 1905-6,) or have the Americanization movement fizzle out before WW1, and get the US to be more strictly neutral in the war or opposing the UK, and then you could presumably keep a nice chunk of the US as effectively bi/tri-lingual. Like Louisiana, the Midwest and much of New England.
But seriously, this is a pretty cool read, even if it's 23 years old:
Language Freedom and Restriction: A Historical Approach to the Official Language Controversy