Fixes a bug introduced in Icf61c9a27fd, which would cause a parser
cache split any time the Cite extension was initialized. The
`setLanguage` interface is regrettable, but I'm hoping it will only
be around temporarily.
Converts an integration test into a unit test and completes coverage.
Bug: T239988
Change-Id: I4b1f8909700845c9fa0cbc1a3de50ee7d42f69a5
