Father in law claims that everybody that emigrated from Europe learned the English language when they became U.S. citizens, I contend otherwise. For example I know there were huge German populations in Pennsylvania that printed everything in German, but did the citizens there still know English?
I can only speak to the local history where I grew up, in south-central Minnesota, to German and Swedish immigrant families. My former church (Lutheran) was founded in 1865 and conducted services exclusively in German. There was a period of mixed-language, and they became exclusively English-using in 1927. This wasn't unusual; in high school we saw a flyer announcing a township government meeting dating from the late 1800's that was printed triligually: French, German, and English.
I've looked into some of this in researching my grandparents; in Chicago during the late 19th / early 20th century, church services, newspapers and parochial schools used Polish in preference to English. According to this source, there was pressure both to preserve Polish and to switch the children, at least, to English.
German here. I remember watching a documentary on this topic. There are some small pockets in the US where the older generation (50+) are still native speakers of German, despite having been born in the US and being the grandchildren of immigrants.
I think for German WWI and WWII were the final death knells for German as a language in the US, during the wars it simply fell out of favour. Newspapers that used to be printed in German either stopped publication or switched over to English.
That during WWII people ended up in internment camps simply for having German heritage sure didn't help either.