In the late 90s I was on a US project which included a small Dutch software company. The founder told me about Dutch people who had been living in the US for thirty plus years, speaking perfect English, now in nursing homes, who with age or dementia or whatever, can lose their ability to speak English. They revert to their native language only. I've never heard of this as an issue again, but I wondered how this might turn into a serious problem caring for many immigrant people who lose the ability to speak English in their later years.