Just because a sentence is known to exist doesn't mean the attacker can reasonably pick it from the ~10^5000 options. Any information you give an attacker about the nature of a password bounds the space, regardless of the nature of the key.

The point is, unless the attacker *knows* they're looking for english passphrases that are very well defined -- which they don't because nobody has a security policy specifying such a thing -- the fact that a password is a phrase in english does not weaken it at all.