Ah it's kinda normal when you brain gets damaged it will revert to the things that make sense to you and usually that's an earlier language or even one you've been exposed to.

Happens all the time with dementia patients. They'll speak English for 80 years but when they were 5 they might have spoken French. They will latch on to when they were 5 as their brain deteriorates. I assume it's cause these kinds of things occupy less space than the decades of English memories.

Not sure why it happened in this case but it's not unheard of for other ailments.