There is not much you can actually do about this, especially in terms of hiring Deputies from different time zones, that require a second English Language speaker.
Even if there grammar isn't "excellent", it will eventually improve, and I am positive that they would attempt to improve it themselves anyways.
I however do see where your argument is coming from, in terms of events such as Trivia, where grammar is essential. And also in terms of them actually warning players, I mean having a somewhat ungrammatical warning really doesn't sound effective or professional, and it is kind of necessary to sound professional at times.
However, as I said, if their grammar isn't amazing, they will improve, and it will actually help them improve.
I also do hope that you are aiming this towards Deputies only, and not other Staff Positions, as other staff positions do not necessarily require any amazing grammar, and require more knowledge/skill.

I do agree with you in terms of Classic, they have a rather strict staff system, and I do think that it is a rather effective one, however you do have to keep in mind the tremendous difference between the player count of both servers. If you have effective hard working staff, you would only require a few, not many are really needed.

^This is really not necessary.