Very unlikely any adult American of the time, assuming they were educated and raised with American English and primary education in that language as their “milk tongue” would forget how to speak the language.
After reading about Georg Gartner, a German POW who escaped a prison camp in New Mexico to avoid being returned to the Soviet occupied part of Germany, I don’t doubt he could essentially forget English while immersed in another language. Gartner didn’t know English at first and didn’t speak until he felt he could without an accent, and after 40 years when he did “surrender” he had a hard time communicating in German again.