>> Because they could get away with using English most of the time. They were never forced to speak it.

I live in a foreign country and this is my life, while english is not really the best here - I've had literally zero issues with either speaking english or having some help every now and then (like for 'official matters').

>> The stress caused by having to come up with the proper word combination helps immensely. We tend to remember what caused us stress.

I also think this is very accurate, most of the words I know are really set in stone from these kinds of occasions. Some specific event happened related to the word, maybe I misspoke something and it was pointed out. And so on.