i think the point here was that it might be useful to have language redundancy so that brain damage which robs you of one language may leave you with some of your backups

Someone close to me suffered something similar, she lost Spanish but kept the ability to speak Lithuanian (Her native language) shortly before her death, doctor commented that it was something he had seen before

It made treatment complex, because no one knew enough Lithuanian to communicate to the degree necesary

Yes, depending on where the stroke/aneurysm the effect can be totally different. I had one a few years ago, I was still able to chat with my colleagues and the hospital staff in Japanese and English, as well as in French (my native language) with my family back in France. However when looking at texts in French or English, I was recognizing the letters but putting them together I wasn't recognizing the meaning behind them. It took me a few days before being able to decipher them again, but it took me 3 weeks before recovering a decent reading speed. But some blood that had escaped in my skull at that time was blocking some nerves linked to the vision, so I had a dead angle of around 45 degrees on my right side until the blood naturally disappeared.