When you right-click the compose area in Proton Mail, is there a check mark next to "Check Spelling"? If not, can you turn it on by clicking that menu item?

Thank you jscher2000: I right-clicked in the composer box and the Check Spelling box was ticked...however, none of the languages was ticked. I clicked on English (GB) without effect (I have downloaded this dictionary before) so I clicked add dictionary, and added the relevant one. In this dialogue, I also right-clicked, to find that the English (US) language was selected, and that the English (GB) box did not respond, so I again did add dictionary, and am able to use a British English spell checker. Thank you so much jscher2000! A simple solution!! Andrew Corser

I'm glad you were able to fill in all the extra steps.