The editor doesn’t seem to have spell check. I find that surprising.
My external system spell check works for the browser when typing in any form. externul sistum spel chek <- underlined in red for me.
Turned out to be a browser (Chrome) issue. Anyone else afflicted by this can select the text in the editor, right click, and select spell check | check spelling in text fields. You just need to do it once and it will resolve the issue for all text fields.