@gprock I'm having exacly the same problem. I was using the normal opera before with 3 dictionaries for spell checking. Looks like when I merged over to the GX the dictionaries partialy stayed, but aren't working and aren't listed in the settings under languages. I can choose them by RMB on text areas, but the missspelled words aren't getting marked.
Adding or removing them isn't possible from the Opera itself, because the "installed" ones aren't listed and when clicking on add new nothing happens.