Going back to what it was.... They moved from Electron to UWP, and now taking a U-turn?
On Windows 11, the new WebView2-based app's spell-check only functions for the primary Windows display language. It does not detect or use other installed language packs that include the 'Basic typing' feature. For example, with the display language set to English, spell check for Dutch and French does not work, despite those language packs being fully installed in the OS.