I just saw a tweet in greek. Funny thing is, I remember perfectly how to read greek, and a native speaker—I know a few of them—understands me perfectly and "s/he" then knows what I say. I don't! I have lost memory of most, if not all, nouns and verbs meaning.
I could become the perfect reader for secret messages. Sometimes brain and memory works in a funny, peculiar way. Why did I keep this knowledge, which I do not practice at all? Do you have funny, twisted 'functions' with your brain and memory you cannot fully explain?
As soon as I see a greek word, I hear the sound in my mind and can read it aloud the same way I can with the languages I speak, Italian, English, and French.
Well, it seems that—most of the times—I understand those languages, too. Ok, sometimes, even. It happened I failed to understand grocery lists, in whatever language, but that is another story...