All those previous comments are few months old. At that time my Aria translation worked fine. Opera's UI-language was English but in language settings the first one was Finnish.
Few Opera's updates later, Aria translation started to behave just like those guys abova tell. English into English. And nothing helped.
Although it asks 'What language you want the translation into?'
But there is no way to set this up. Still Finnish the 1st language in the settings.
It works fine eg. from German to English.
So there must be a bug in Opera. And in my experience it never will be fixed. That's Opera's policy. Good example is that video problem too.
I've been using Opera for so long, both in my laptop and phone. Hoping that bugs would be fixed some day. I'll give up now, that will never happen and that's a pity.