I'm reading this section:
What a private language is
"If someone were to behave as if they understood a language of which no one else can make sense, we might call this an example of a private language. It is not sufficient here, however, for the language to simply be one that has not yet been translated. In order to count as a private language in Wittgenstein's sense, it must be in principle incapable of translation into an ordinary language – if for example it were to describe those inner experiences supposed to be inaccessible to others. The private language being considered is not simply a language in fact understood by one person, but a language that in principle can only be understood by one person. So the last speaker of a dying language would not be speaking a private language, since the language remains in principle learnable. A private language must be unlearnable and untranslatable, and yet it must appear that the speaker is able to make sense of it."
I'm having trouble seeing what "in principle can only be understood by one person" means. Suppose all humans have died except one. He understands English but now he can't write or speak anymore. Let's say all records of English have been destroyed. The last human just relies on his memory of English for his own internal thought process. Would English in this situation count as a private language in Wittgenstein's terms?