The official help just says that there is spell checking context menu for edit fields. Yes I see "spell checking" on context menu.
But I see no red underlines to mark misspelled words as I write words. When I click "spell checking" on context menu, it apparently does nothing, I'm not sure what it's supposed to do: toggle spell checking mode (red underlines) on and off, or popup something like a find-and-replace dialog box?
It seems I was confused because my installation of Opera was broken with spell-checking feature (Opera 10.01 (current stable version) on Ubuntu 8.10, installed with the default package from the Opera homepage). Right-clicking on a textarea didn't show "Dictionaries" item, for example.
But after I installed Opera 10.10 beta (again with the default beta package from the Opera homepage), it works and makes sense. Red underlines appear on misspells and there is "Dictionaries" item.