Wavebox now incorporates the full Chromium-based spell checker, allowing you to have multiple languages and add custom words into the spell checker as well.

The settings location has changed in Wavebox 10- you can get there in Settings by clicking on "More Browser Settings" in the Advanced section.

This will bring up a new settings window - if you click on Advanced there and then scroll down, you'll see the full Languages section allowing you to add further languages and configure the spell check as you wish.