As you'd expect from Google, the Material writing guide focuses on writing superb interface copy, with a fairly standard emphasis on clarity, concision, accessibility, and direct address. Note that Google calls out the basis for their style guide, the Associated Press, which is a smart way to cover topics you don't intend to include in your own guide.
Text should be understandable by anyone, anywhere, regardless of their culture or language.
Clear, accurate, and concise text makes interfaces more usable and builds trust.
Google guidelines generally follow the Associated Press (AP) style guidelines.