Some of us with German immigrant ancestry know this isn't a myth. My dad was born in 1904, and raised in a Central Washington town named Ruff. (Pronounced "roof") The entire town was Germans, and he said he never spoke English until WWI began. At the beginning of WWI the county sheriff went around to all the German farmers and told them they had a sundown curfew, and weren't allowed out after dark. At the same time the German farmers made a big effort to not speak German around anyone who was an "outsider" and to teach all the kids to speak English also.
When I was a kid my friends would ask me how long my dad had been in the US? I told them he was born here, and got the strangest looks from my friends, who assumed with his German accent he was an immigrant here.

So true. My German Grandparents Americanized their last name to Miller from Mueller back in Michigan. My grand dad spoke English while at work as a blacksmith city employee. During WW1 the community was worried about a domestic German uprising, so key employees where given a Trapdoor Springfield rifle, bayonet and ammo to prevent such "foolishness". The rifle was never returned and I still have it.
kw

KFW wrote on Dec 15^th, 2021 at 1:35pm:


My grandfather was also a Mueller, and changed it to Miller! My dad said it was pronounced "Meeller" when it was spelled Mueller. Had umlauts over the "u" which made it a different pronunciation. 
My dad and my uncle had it changed back to Mueller (the original spelling) on his headstone many years after he died.

yep!  My Grandfather was born in in Freidheim, MO about 1895.
During WWI they changed the name to Freetown.  Afterwards, they changed it back about 1923/4?

Rich