I've found in the past that the browser must have the
dictionary/language installed corresponding to the language the page
specifies, e.g. with sites in en-US my en-GB Firefox wouldn't
check spelling on Microsoft Windows.
Atto works for me with Firefox 76.0 on Fedora 30:
And Chrome 88.0.4324.182:
(This did mean I ran into a Fedora bug: Firefox defaults to English (Malawi) which is a bit odd).