G2G uses CKEditor, but I can't tell which version: 4, which runs its own spelling checker, or 5, which AFAICT does not. In either case, it looks like someone at WikiTree needs to look at the settings they're using, because it currently hard-codes the editor to the user's default locale, and doesn't allow on-the-fly switching of languages (or, rather, ignores them completely as far as the spelling checking is concerned).