Putting aside for the moment the fact that changing the locale does not in fact work (it seems this setting is hardcoded from the locale at install time, and I am not reinstalling), changing the locale of the OS interferes with other localized applications when Opera already has a setting for this which is being completely ignored. It's not a reasonable solution.
I am using the second suggestion already, a custom search with a keyword. However I have been using the keyword for wikipedia in opera for years and it is quite difficult to change those old habits.
I'm not looking for ways to cope with the problem, I'm looking for ways to fix it.
Or in the very least, making sure someone is aware that it is currently broken.