This is precisely what I've been hoping somebody would build. In my initial testing, it works well. I can even mix sentences with different languages, and it still makes correct suggestions.

The fluency suggestions are seemingly largely malfunctioning. It frequently suggests starting and ending sentences with quotes, although it also makes some useful suggestions. There seems to be an issue with analysis running synchronized, or something like that; when I type into a text field and Refine starts to run, it often blocks text entry. Selecting a suggested replacement blocks the app for half a second or so. Neither of these problems occurs with Grammarly or Language Tool. I also noticed a bunch of issues that Grammarly catches (like verb agreement) that Refine does not.

But this is an amazing first release and extremely promising. Congrats!