Persistence and a keen desire to learn what's working and what's not working on a platform. That's all you need. I don't think you need a strong grip on your language. In fact, people like you more if you make occasional mistakes in your writing because it looks more authentic.
It depends on the kind of content creator. As a technical copywriter, I believe these are the important skills to have:
* Capacity to thinking clearly and putting your structured thoughts on paper
* Great eye for simplicity in your words
* Always performing great research for everything that is written
* Great capacity to comeback to your piece and remove all the clutter of the first draft.