You can't blame this one on English.

Before the ambiguity of language can get in the way, there has to be a coherent idea that you want to express in the first place.

This license explicitly contradicts itself. It says you are encouraged to contribute changes to the source, and you may not share changes to the source with anyone ever.

It seems like the intent is to encourage giving changes to exclusively the maintainer, and that forking in this context refers to distribution beyond a private communication between the change proposer and the maintainer.

Which, this being a git repo, makes perfect sense, as despite the impression GitHub gave to a whole generation of developers, Git was designed with a different contribution workflow in mind: one of sending patches by e-mail. Cloning the repo locally, making changes, formatting a patch, and sending it to the maintainer, seems to be perfectly withing bounds of both intent and text of the license.

Not really. Legalese is vague on purpose because not every situation can be rigidly defined.

As for English, because of its plain nature, I have little trouble understanding someone who isn't proficient or who has a heavy accent, whereas languages with specific infections or tones might not have that kind of liberty.