Hi Thomas.
Welcome to Affinity Forums Go to System Preferences > Keyboard section, Text tab (on top), click the Spelling dropdown on the right and select the last option Set Up.... Add/tick U.S. English and press Done. Restart Affinity Publisher. A new English option should appear in the Language dropdown now. It has no locale/region specified - i mean it appears just as English in the list - but should work correctly: it's US English (the missing locale issue was already logged and should be fixed soon).
