“But, assuming you believe his memoirs, when they spoke to him, he regained his English within a day.”
I think this works if the language is your native language or if you have immersed yourself. I used to be ok with French but after a few years of not using it there is not much left.
You’d be amazed how fast it comes back. I hadn’t used French in over a decade last time I went to France. In the taxi to our hotel I could barely string two words together. Ten days later in the one to the airport I was having a real conversation, if with terrible grammar and poor vocabulary.
When I was coming back to Denmark I did some reading on academic studies related to relearning languages, one study I found said that if someone has spoken a language for 8 years (or the first 8 years of their life as it was focused on childhood language being relearned in adulthood) then they will never lose that language - unfortunately I spoke Danish the first 6 years of my life and when I came back I started to remember German which occupied the part of my mind focused on non-English language.
I think this works if the language is your native language or if you have immersed yourself. I used to be ok with French but after a few years of not using it there is not much left.