Windows, Opera GX LVL2 (core: 73.0.3856.424)
So, there seems to be a problem lately with spellchecking (3-4 patches now), it used to work correctly in the past.
When I have it on "All your languages", it only detects the spelling problems in latin alphabet languages (in my case English and French) but it does not do the same for Greek anymore(I don't know if the problem is the same with the rest of the alphabets like Chinese, Arabic etc.)
However, when I have it only in Greek, it does work correctly. So the problem is not that it doesn't detect Greek spelling errors in general, it's just that it doesn't recognise them when it's on "All your languages".
This is a snapshot when I have "All your languages". As you can see, French and English are detected, but Greek is not:
And this is a snapshot of "Greek" as the spellchecking language, and here you can see that the words are now recognised as mistakes: