Well, it's not an instantaneous translation like the feature you're suggesting would be, but the "Translator" extension on the front page of Opera's extensions site acts quite similar to what you're suggesting (https://addons.opera.com/en/extensions/details/translator/).
If you highlight a block of text, all you have to do is click on the Translator icon in your toolbar and it will automatically paste in the text, auto-detect the language, and translate it for you. It's actually quite good. So like I mentioned, it's not instantaneous translation like you want but it's really only one quick extra step.