This is not the first or only time something like this has happened. It makes one wonder if there is some type of latent ability within the brain to be able to understand other languages that we consciously are unable to. There are also other examples of other people who have experienced brain "damage" that enabled them to play instruments or even have an overwhelming urge to create art. It's not strange at all for someone who has a stroke or hits their head to be debilitated mentally - but for someone to have mental abilities "unlocked" is obviously a lot more mysterious.
This is different from Foreign Accent Syndrome which is when someone starts to seemingly have their accent change after a similar incident. That is from damages to the parts of the brain that control how we talk in our own language - that causes it coincidentally to sound like a "Foreign" or different accent that the speaker did originally. To be able to speak another language you didn't previously - and forget your original language is a totallly different story!
Englishman wakes up from stroke speaking fluent Welsh
An Englishman who suffered a stroke has surprised doctors after he woke up speaking fluent Welsh. Link
Alun Morgan, 81, was evacuated to Wales during the Second World War but left 70 years ago. During his time there he was surrounded by Welsh speakers but never learned the language himself. He left the country aged 10 and lived his life in England and recently suffered a severe stroke. But when Mr Morgan regained consciousness three weeks later, doctors discovered he was speaking Welsh and could not remember any English.
It is thought that the Welsh Mr Morgan heard as a boy had sunk in without him knowing and was unlocked after he suffered the stroke.