Notejoy's web app leverages your browser's built-in spell checker to offer spelling corrections. So look for your browser's settings to customize it.
Notejoy's Mac app leverages our own spell checker. You'll notice red squiggles underlining potentially misspelled words, which you can right-click on to get and apply spelling corrections. Pressing the spell check icon on the styling bar will re-run the spell checker across your note and refresh the red squiggles. It defaults to the language preference set on your operating system. So simply change your operating system's language setting to customize Notejoy's spell checker language. The app supports all languages supported by Google Chrome.
Notejoy's Windows app does not currently support spell checking, but it's coming soon. Upvote it here to be notified when it is available.