Jami-client-gnome (jami-20200507.3.7fe46ae-1.fc31) does not take advantage of the system's spell checker (e.g. hunspell is used by GTK, Qt, LibreOffice, Firefox, Chrome, etc and can be considered as GNU/Linux's default spell checking provider).

Consider using hunspell for the chat text editor field.

Hunspell: http://hunspell.github.io/