Hey Martin, You can definitely publish in Czech! It's just that other Hashnode users don't understand Czech and probably won't get any value out of it. That's the reason we have enabled an option in the editor (hashnode.com/create/story) which lets you exclude certain posts from the Hashnode community while keeping them on your blog. So, you can keep blogging in Czech and start building your audience. It's just that the posts won't pop up in the Hashnode users' feeds.
In the future, we can auto-translate the posts into English for our English speaking audience and maybe even build a Czech community inside Hashnode. :)