Hopefully Opera’s known power user, Tamil, had a solution for a similar task, which I changed a bit:

1. Go to Tools -> Preferences -> Advanced -> Toolbar
2. Find “Menu Setup” (bottom right), click “Opera Standard” and press “Duplication” (skip this part if you already have a “Copy of Opera Standard”).
3. From the profile directory, open “menu” folder and edit the file called “standard menu (1).ini”
4. Find the section named “[Hotclick Popup Menu]” and add the entry:

   Item, "Google Auto Translation"= Go to page, "http://translate.google.com/?hl=auto#auto|auto|%t"

5. Save the file, restart Opera

Now simply select a word or phrase from the HTML document, right click and choose “Google Auto Translation”.

If you want to have the ability to translate the entire page, find the section named “[Document Popup Menu]” and add the following item:

Item, "Google Auto Translation"= Go to page, "http://translate.google.com/translate?hl=auto&sl=auto&tl=auto&u=%u"