Substitute `apt` with your distro's package manager. In Ghostwriter, go to Settings > Preferences > Spell Check (tab), and select the desired language. You might need to toggle the spellcheck or switch the language back and forth for it to work.
Here is a list of Hunspell dictionaries in Ubuntu (18.04) repo:
**Edit (06/06/2019)**: Recently discovered [Inkdrop](https://inkdrop.app/) through [HN](https://news.ycombinator.com/item?id=20103589). It's a note-taking app that supports Markdown and can sync across devices. I think the best feature is its ability to sync the scrolling in editor and HTML output, which is absent in Ghostwriter. I only need a simple editor for my current use case, though I might consider it when I have >50 posts.
Reading through the HN comments, I also discovered [Visual Studio Code](https://code.visualstudio.com/) also supports Markdown and HTML preview. For a Markdown editor, it's probably overkill though.
