It is not uncommon for someone who has had a stroke to lose their second language(s) but not their first. I have even had a patient who had not spoken French since age 5 (moved to an English speaking area) and whose English was indistinguishable from the English only speakers around her. She woke up from a stroke fluent in French but with no knowledge of English. This suggests to me that there is actually a different area of the brain involved.

At almost 70 she started English classes and her husband of 45 years started learning French.

Pook, you mentioned that anyone can learn not to have an accent. Anyone can learn to sing opera too. But it will be a LOT harder for some people than others. One of the problems in language and especially accent learning is that we lose in childhood the ability to distinguish sounds that we do not hear. Is phoneme the work I want for the sounds in a language? Some people have perfect pitch and others have to work at it and others are tone deaf. There is also "an ear for language". The more polyglot the surroundings a child is raised in the better their "ear" will be and the better that child will be at hearing and therefore being able to reproduce foreign languages accurately.